PERF: Eager load Theme associations in Stylesheet Manager.

Before this change, calling `StyleSheet::Manager.stylesheet_details`
for the first time resulted in multiple queries to the database. This is
because the code was modelled in a way where each `Theme` was loaded
from the database one at a time.

This PR restructures the code such that it allows us to load all the
theme records in a single query. It also allows us to eager load the
required associations upfront. In order to achieve this, I removed the
support of loading multiple themes per request. It was initially added
to support user selectable theme components but the feature was never
completed and abandoned because it wasn't a feature that we thought was
worth building.

app/helpers/application_helper.rb

@@ -408,14 +408,19 @@ module ApplicationHelper
     end
   end
 
-  def theme_ids
+  def theme_id
     if customization_disabled?
-      [nil]
+      nil
     else
-      request.env[:resolved_theme_ids]
+      request.env[:resolved_theme_id]
     end
   end
 
+  def stylesheet_manager
+    return @stylesheet_manager if defined?(@stylesheet_manager)
+    @stylesheet_manager = Stylesheet::Manager.new(theme_id: theme_id)
+  end
+
   def scheme_id
     return @scheme_id if defined?(@scheme_id)
 
@@ -424,12 +429,9 @@ module ApplicationHelper
       return custom_user_scheme_id
     end
 
-    return if theme_ids.blank?
+    return if theme_id.blank?
 
-    @scheme_id = Theme
-      .where(id: theme_ids.first)
-      .pluck(:color_scheme_id)
-      .first
+    @scheme_id = Theme.where(id: theme_id).pluck_first(:color_scheme_id)
   end
 
   def dark_scheme_id
@@ -457,7 +459,7 @@ module ApplicationHelper
 
   def theme_lookup(name)
     Theme.lookup_field(
-      theme_ids,
+      theme_id,
       mobile_view? ? :mobile : :desktop,
       name,
       skip_transformation: request.env[:skip_theme_ids_transformation].present?
@@ -466,7 +468,7 @@ module ApplicationHelper
 
   def theme_translations_lookup
     Theme.lookup_field(
-      theme_ids,
+      theme_id,
       :translations,
       I18n.locale,
       skip_transformation: request.env[:skip_theme_ids_transformation].present?
@@ -475,7 +477,7 @@ module ApplicationHelper
 
   def theme_js_lookup
     Theme.lookup_field(
-      theme_ids,
+      theme_id,
       :extra_js,
       nil,
       skip_transformation: request.env[:skip_theme_ids_transformation].present?
@@ -483,22 +485,26 @@ module ApplicationHelper
   end
 
   def discourse_stylesheet_link_tag(name, opts = {})
-    if opts.key?(:theme_ids)
-      ids = opts[:theme_ids] unless customization_disabled?
-    else
-      ids = theme_ids
-    end
+    manager =
+      if opts.key?(:theme_id)
+        Stylesheet::Manager.new(
+          theme_id: customization_disabled? ? nil : opts[:theme_id]
+        )
+      else
+        stylesheet_manager
+      end
 
-    Stylesheet::Manager.stylesheet_link_tag(name, 'all', ids)
+    manager.stylesheet_link_tag(name, 'all')
   end
 
   def discourse_color_scheme_stylesheets
     result = +""
-    result << Stylesheet::Manager.color_scheme_stylesheet_link_tag(scheme_id, 'all', theme_ids)
+    result << stylesheet_manager.color_scheme_stylesheet_link_tag(scheme_id, 'all')
 
     if dark_scheme_id != -1
-      result << Stylesheet::Manager.color_scheme_stylesheet_link_tag(dark_scheme_id, '(prefers-color-scheme: dark)', theme_ids)
+      result << stylesheet_manager.color_scheme_stylesheet_link_tag(dark_scheme_id, '(prefers-color-scheme: dark)')
     end
+
     result.html_safe
   end
 
@@ -525,7 +531,7 @@ module ApplicationHelper
       asset_version: Discourse.assets_digest,
       disable_custom_css: loading_admin?,
       highlight_js_path: HighlightJs.path,
-      svg_sprite_path: SvgSprite.path(theme_ids),
+      svg_sprite_path: SvgSprite.path(theme_id),
       enable_js_error_reporting: GlobalSetting.enable_js_error_reporting,
       color_scheme_is_dark: dark_color_scheme?,
       user_color_scheme_id: scheme_id,
@@ -533,7 +539,7 @@ module ApplicationHelper
     }
 
     if Rails.env.development?
-      setup_data[:svg_icon_list] = SvgSprite.all_icons(theme_ids)
+      setup_data[:svg_icon_list] = SvgSprite.all_icons(theme_id)
 
       if ENV['DEBUG_PRELOADED_APP_DATA']
         setup_data[:debug_preloaded_app_data] = true