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

spec/lib/onebox/engine/github_pullrequest_onebox_spec.rb

@@ -46,8 +46,9 @@ describe Onebox::Engine::GithubPullRequestOnebox do
       expect(html).to include("1")
     end
 
-    it "includes the body" do
+    it "includes the body without comments" do
       expect(html).to include("http://meta.discourse.org/t/audio-html5-tag/8168")
+      expect(html).not_to include("test comment")
     end
   end
 end