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.

spec/requests/qunit_controller_spec.rb

@@ -0,0 +1,76 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+describe QunitController do
+  let(:theme) { Fabricate(:theme, name: 'main-theme') }
+  let(:component) { Fabricate(:theme, component: true, name: 'enabled-component') }
+  let(:disabled_component) { Fabricate(:theme, component: true, enabled: false, name: 'disabled-component') }
+
+  before do
+    Theme.destroy_all
+    theme.set_default!
+    component.add_relative_theme!(:parent, theme)
+    disabled_component.add_relative_theme!(:parent, theme)
+    [theme, component, disabled_component].each do |t|
+      t.set_field(
+        target: :extra_js,
+        type: :js,
+        name: "discourse/initializers/my-#{t.id}-initializer.js",
+        value: "console.log(#{t.id});"
+      )
+      t.set_field(
+        target: :tests_js,
+        type: :js,
+        name: "acceptance/some-test-#{t.id}.js",
+        value: "assert.ok(#{t.id});"
+      )
+      t.save!
+    end
+  end
+
+  context "when no theme is specified" do
+    it "includes tests of enabled theme + components" do
+      get '/qunit'
+      js_urls = JavascriptCache.where(theme_id: [theme.id, component.id]).map(&:url)
+      expect(js_urls.size).to eq(2)
+      js_urls.each do |url|
+        expect(response.body).to include(url)
+      end
+      [theme, component].each do |t|
+        expect(response.body).to include("/theme-javascripts/tests/#{t.id}.js")
+      end
+
+      js_urls = JavascriptCache.where(theme_id: disabled_component).map(&:url)
+      expect(js_urls.size).to eq(1)
+      js_urls.each do |url|
+        expect(response.body).not_to include(url)
+      end
+      expect(response.body).not_to include("/theme-javascripts/tests/#{disabled_component.id}.js")
+    end
+  end
+
+  context "when a theme is specified" do
+    it "includes tests of the specified theme only" do
+      [theme, disabled_component].each do |t|
+        get "/qunit?theme_name=#{t.name}"
+        js_urls = JavascriptCache.where(theme_id: t.id).map(&:url)
+        expect(js_urls.size).to eq(1)
+        js_urls.each do |url|
+          expect(response.body).to include(url)
+        end
+        expect(response.body).to include("/theme-javascripts/tests/#{t.id}.js")
+
+        excluded = Theme.pluck(:id) - [t.id]
+        js_urls = JavascriptCache.where(theme_id: excluded).map(&:url)
+        expect(js_urls.size).to eq(2)
+        js_urls.each do |url|
+          expect(response.body).not_to include(url)
+        end
+        excluded.each do |id|
+          expect(response.body).not_to include("/theme-javascripts/tests/#{id}.js")
+        end
+      end
+    end
+  end
+end