DEV: Simplify ember-cli proxy strategy (#24242)

Previously, the app HTML served by the Ember-CLI proxy was generated based on a 'bootstrap json' payload generated by Rails. This inevitably leads to differences between the Rails HTML and the Ember-CLI HTML.

This commit overhauls our proxying strategy. Now, we totally ignore the ember-cli `index.html` file. Instead, we take the full HTML from Rails and surgically replace script URLs based on a `data-discourse-entrypoint` attribute. This should be faster (only one request to Rails), more robust, and less confusing for developers.

app/controllers/bootstrap_controller.rb

@@ -1,103 +1,8 @@
 # frozen_string_literal: true
 
 class BootstrapController < ApplicationController
-  include ApplicationHelper
-
   skip_before_action :redirect_to_login_if_required, :check_xhr
 
-  # This endpoint allows us to produce the data required to start up Discourse via JSON API,
-  # so that you don't have to scrape the HTML for `data-*` payloads
-  def index
-    locale = script_asset_path("locales/#{I18n.locale}")
-
-    preload_anonymous_data
-    if current_user
-      current_user.sync_notification_channel_position
-      preload_current_user_data
-    end
-
-    @stylesheets = []
-
-    add_scheme(scheme_id, "all", "light-scheme")
-    add_scheme(dark_scheme_id, "(prefers-color-scheme: dark)", "dark-scheme")
-
-    if rtl?
-      add_style(mobile_view? ? :mobile_rtl : :desktop_rtl)
-    else
-      add_style(mobile_view? ? :mobile : :desktop)
-    end
-    add_style(rtl? ? :admin_rtl : :admin) if staff?
-    add_style(rtl? ? :wizard_rtl : :wizard) if admin?
-
-    assets_fake_request = ActionDispatch::Request.new(request.env.dup)
-    assets_for_url = params[:for_url]
-    if assets_for_url
-      path, query = assets_for_url.split("?", 2)
-      assets_fake_request.env["PATH_INFO"] = path
-      assets_fake_request.env["QUERY_STRING"] = query
-    end
-
-    Discourse
-      .find_plugin_css_assets(
-        include_official: allow_plugins?,
-        include_unofficial: allow_third_party_plugins?,
-        mobile_view: mobile_view?,
-        desktop_view: !mobile_view?,
-        request: assets_fake_request,
-        rtl: rtl?,
-      )
-      .each { |file| add_style(file, plugin: true) }
-    add_style(mobile_view? ? :mobile_theme : :desktop_theme) if theme_id.present?
-
-    extra_locales = []
-    if ExtraLocalesController.client_overrides_exist?
-      extra_locales << ExtraLocalesController.url("overrides")
-    end
-
-    extra_locales << ExtraLocalesController.url("admin") if staff?
-
-    extra_locales << ExtraLocalesController.url("wizard") if admin?
-
-    plugin_js =
-      Discourse
-        .find_plugin_js_assets(
-          include_official: allow_plugins?,
-          include_unofficial: allow_third_party_plugins?,
-          request: assets_fake_request,
-        )
-        .map { |f| script_asset_path(f) }
-
-    plugin_test_js =
-      if Rails.env != "production"
-        script_asset_path("plugin-tests")
-      else
-        []
-      end
-
-    bootstrap = {
-      theme_id: theme_id,
-      theme_color: "##{ColorScheme.hex_for_name("header_background", scheme_id)}",
-      title: SiteSetting.title,
-      current_homepage: current_homepage,
-      locale_script: locale,
-      stylesheets: @stylesheets,
-      plugin_js: plugin_js,
-      plugin_test_js: plugin_test_js,
-      setup_data: client_side_setup_data,
-      preloaded: @preloaded,
-      html: create_html,
-      theme_html: create_theme_html,
-      html_classes: html_classes,
-      html_lang: html_lang,
-      login_path: main_app.login_path,
-      authentication_data: authentication_data,
-    }
-    bootstrap[:extra_locales] = extra_locales if extra_locales.present?
-    bootstrap[:csrf_token] = form_authenticity_token if current_user
-
-    render_json_dump(bootstrap: bootstrap)
-  end
-
   def plugin_css_for_tests
     urls =
       Discourse
@@ -114,74 +19,4 @@ class BootstrapController < ApplicationController
 
     render plain: stylesheet, content_type: "text/css"
   end
-
-  private
-
-  def add_scheme(scheme_id, media, css_class)
-    return if scheme_id.to_i == -1
-
-    if style =
-         Stylesheet::Manager.new(theme_id: theme_id).color_scheme_stylesheet_details(
-           scheme_id,
-           media,
-         )
-      @stylesheets << { href: style[:new_href], media: media, class: css_class }
-    end
-  end
-
-  def add_style(target, opts = nil)
-    if styles = Stylesheet::Manager.new(theme_id: theme_id).stylesheet_details(target, "all")
-      styles.each do |style|
-        @stylesheets << {
-          href: style[:new_href],
-          media: "all",
-          theme_id: style[:theme_id],
-          target: style[:target],
-        }.merge(opts || {})
-      end
-    end
-  end
-
-  def create_html
-    html = {}
-    return html unless allow_plugins?
-
-    add_plugin_html(html, :before_body_close)
-    add_plugin_html(html, :before_head_close)
-    add_plugin_html(html, :before_script_load)
-    add_plugin_html(html, :header)
-
-    html
-  end
-
-  def add_plugin_html(html, key)
-    add_if_present(
-      html,
-      key,
-      DiscoursePluginRegistry.build_html("server:#{key.to_s.dasherize}", self),
-    )
-  end
-
-  def create_theme_html
-    theme_html = {}
-    return theme_html if customization_disabled?
-
-    theme_view = mobile_view? ? :mobile : :desktop
-
-    add_if_present(theme_html, :body_tag, Theme.lookup_field(theme_id, theme_view, "body_tag"))
-    add_if_present(theme_html, :head_tag, Theme.lookup_field(theme_id, theme_view, "head_tag"))
-    add_if_present(theme_html, :header, Theme.lookup_field(theme_id, theme_view, "header"))
-    add_if_present(
-      theme_html,
-      :translations,
-      Theme.lookup_field(theme_id, :translations, I18n.locale),
-    )
-    add_if_present(theme_html, :js, Theme.lookup_field(theme_id, :extra_js, nil))
-
-    theme_html
-  end
-
-  def add_if_present(hash, key, val)
-    hash[key] = val if val.present?
-  end
 end