FEATURE: Introduce theme/component QUnit tests (#12517)

This commit allows themes and theme components to have QUnit tests. To add tests to your theme/component, create a top-level directory in your theme and name it `test`, and Discourse will save all the files in that directory (and its sub-directories) as "tests files" in the database. While tests files/directories are not required to be organized in a specific way, we recommend that you follow Discourse core's tests [structure](https://github.com/discourse/discourse/tree/master/app/assets/javascripts/discourse/tests).

Writing theme tests should be identical to writing plugins or core tests; all the `import` statements and APIs that you see in core (or plugins) to define/setup tests should just work in themes.

You do need a working Discourse install to run theme tests, and you have 2 ways to run theme tests:

* In the browser at the `/qunit` route. `/qunit` will run tests of all active themes/components as well as core and plugins. The `/qunit` now accepts a `theme_name` or `theme_url` params that you can use to run tests of a specific theme/component like so: `/qunit?theme_name=<your_theme_name>`.

* In the command line using the `themes:qunit` rake task. This take is meant to run tests of a single theme/component so you need to provide it with a theme name or URL like so: `bundle exec rake themes:qunit[name=<theme_name>]` or `bundle exec rake themes:qunit[url=<theme_url>]`.

There are some refactors to internal code that's responsible for processing themes/components in Discourse, most notably:

* `<script type="text/discourse-plugin">` tags are automatically converted to modules.

* The `theme-settings` service is removed in favor of a simple `lib` file responsible for managing theme settings. This was done to allow us to register/lookup theme settings very early in our Ember app lifecycle and because there was no reason for it to be an Ember service.

These refactors should 100% backward compatible and invisible to theme developers.

app/controllers/theme_javascripts_controller.rb

@@ -8,7 +8,7 @@ class ThemeJavascriptsController < ApplicationController
     :preload_json,
     :redirect_to_login_if_required,
     :verify_authenticity_token,
-    only: [:show]
+    only: [:show, :show_tests]
   )
 
   before_action :is_asset_path, :no_cookies, :apply_cdn_headers, only: [:show]
@@ -34,6 +34,29 @@ class ThemeJavascriptsController < ApplicationController
     send_file(cache_file, disposition: :inline)
   end
 
+  def show_tests
+    raise Discourse::NotFound if Rails.env.production?
+
+    theme_id = params.require(:theme_id)
+    theme = Theme.find(theme_id)
+    content = ThemeField
+      .where(
+        theme_id: theme_id,
+        target_id: Theme.targets[:tests_js]
+      )
+      .each(&:ensure_baked!)
+      .map(&:value_baked)
+      .join("\n")
+
+    ThemeJavascriptCompiler.force_default_settings(content, theme)
+
+    response.headers["Content-Length"] = content.size.to_s
+    response.headers["Last-Modified"] = Time.zone.now.httpdate
+    immutable_for(1.second)
+
+    send_data content, filename: "js-tests-theme-#{theme_id}.js", disposition: :inline
+  end
+
   private
 
   def query