Improve support for old browsers (#9515)

* FEATURE: Improve crawler view

* FIX: Make lazyYT crawler-friendly

* DEV: Rename discourse-internet-explorer to discourse-unsupported-browser

* DEV: Detect more unsupported browsers

Follow-up to 4eebbd221239eaa3fc7e6f1bd6a9dcf0239b8444.

* FIX: Hide browser update notice in print view

plugins/discourse-unsupported-browser/plugin.rb

@@ -0,0 +1,52 @@
+# frozen_string_literal: true
+
+# name: discourse-unsupported-browser
+# about: Attempts to provide support for old and unsupported browser through polyfills
+# version: 1.0
+# authors: Joffrey Jaffeux, David Taylor, Daniel Waterworth, Robin Ward
+# url: https://github.com/discourse/discourse/tree/master/plugins/discourse-unsupported-browser
+
+enabled_site_setting :discourse_unsupported_browser_enabled
+hide_plugin if self.respond_to?(:hide_plugin)
+
+register_asset 'stylesheets/ie.scss'
+
+# We can't use register asset for an optional resource. Instead copy it after plugins have
+# been activated so it can be uploaded to CDNs.
+DiscourseEvent.on(:after_plugin_activation) do ||
+  polyfill_path = "#{Plugin::Instance.js_path}/#{self.directory_name}-optional.js"
+  FileUtils.cp("#{Rails.root}/public/plugins/discourse-unsupported-browser/js/ie.js", polyfill_path)
+  Rails.configuration.assets.precompile << "plugins/discourse-unsupported-browser-optional.js"
+end
+
+after_initialize do
+  # Conditionally load the stylesheet
+  register_asset_filter do |type, request|
+    request.nil? || request.env['HTTP_USER_AGENT'] =~ /MSIE|Trident/
+  end
+
+  register_anonymous_cache_key(:ie) do
+    unless defined?(@is_ie)
+      session = @env[self.class::RACK_SESSION]
+      # don't initialize params until later
+      # otherwise you get a broken params on the request
+      params = {}
+
+      @is_ie = BrowserDetection.browser(@env[self.class::USER_AGENT]) == :ie
+    end
+
+    @is_ie
+  end
+
+  # not using patch on preload_script as js is fine and we need this file
+  # to be loaded before other files
+  register_html_builder('server:before-script-load') do |controller|
+    if BrowserDetection.browser(controller.request.env['HTTP_USER_AGENT']) == :ie
+      path = controller.helpers.script_asset_path('plugins/discourse-unsupported-browser-optional')
+
+      <<~JAVASCRIPT
+        <script src="#{path}"></script>
+      JAVASCRIPT
+    end
+  end
+end