FEATURE: Allow theme tests to be run in production (#12815)

This commit allows site admins to run theme tests in production via a new `/theme-qunit` route. When you visit `/theme-qunit`, you'll see a list of the themes/components installed on your site that have tests, and from there you can select a theme or component that you run its tests.

We also have a new rake task `themes:install_and_test` that can be used to install a list of themes/components on a temporary database and run the tests of the themes/components that are installed. This rake task can be useful when upgrading/deploying a Discourse instance to make sure that the installed themes/components are compatible with the new Discourse version being deployed, and if the tests fail you can abort the build/deploy process so you don't end up with a broken site.

app/models/theme.rb

@@ -485,6 +485,16 @@ class Theme < ActiveRecord::Base
     end
   end
 
+  def cached_default_settings
+    Discourse.cache.fetch("default_settings_for_theme_#{self.id}", expires_in: 30.minutes) do
+      settings_hash = {}
+      self.settings.each do |setting|
+        settings_hash[setting.name] = setting.default
+      end
+      settings_hash
+    end
+  end
+
   def build_settings_hash
     hash = {}
     self.settings.each do |setting|
@@ -503,6 +513,7 @@ class Theme < ActiveRecord::Base
   def clear_cached_settings!
     DB.after_commit do
       Discourse.cache.delete("settings_for_theme_#{self.id}")
+      Discourse.cache.delete("default_settings_for_theme_#{self.id}")
     end
   end
 
@@ -662,6 +673,23 @@ class Theme < ActiveRecord::Base
     setting_row.save!
   end
 
+  def baked_js_tests_with_digest
+    content = theme_fields
+      .where(target_id: Theme.targets[:tests_js])
+      .each(&:ensure_baked!)
+      .map(&:value_baked)
+      .join("\n")
+
+    return [nil, nil] if content.blank?
+
+    content = <<~JS + content
+      (function() {
+        require("discourse/lib/theme-settings-store").registerSettings(#{self.id}, #{cached_default_settings.to_json}, { force: true });
+      })();
+    JS
+    [content, Digest::SHA1.hexdigest(content)]
+  end
+
   private
 
   def to_scss_variable(name, value)