Various GitHub Onebox improvements (#13163)

* FIX: Improve GitHub folder regexp in Onebox

It used to match any GitHub URL that was not matched by the other GitHub
Oneboxes and it did not do a good job at handling those. With this
change, the generic Onebox will handle the remaining URLs.

* FEATURE: Add Onebox for GitHub Actions

* FEATURE: Add Onebox for PR check runs

* FIX: Remove image from GitHub folder Oneboxes

It is a generic, auto-generated image which does not provide any value.

* DEV: Add tests

* FIX: Strip HTML comments from PR body

lib/onebox/engine/github_folder_onebox.rb

@@ -7,14 +7,9 @@ module Onebox
       include StandardEmbed
       include LayoutSupport
 
-      matches_regexp(/^https?:\/\/(?:www\.)?(?:(?:\w)+\.)?(github)\.com[\:\d]*(\/[^\/]+){2}/)
+      matches_regexp(/^https?:\/\/(?:www\.)?(?:(?:\w)+\.)?(github)\.com[\:\d]*(\/[^\/]+){2}\/tree/)
       always_https
 
-      def self.priority
-        # This engine should have lower priority than the other Github engines
-        150
-      end
-
       private
 
       def data
@@ -46,7 +41,6 @@ module Onebox
 
         {
           link: url,
-          image: og.image,
           title: Onebox::Helpers.truncate(title, 250),
           path: display_path,
           description: display_description,