Add GitHub Required Status Checks compatability

Uses fixed context of 'gitlab/ci' instead of including the refname.
This can introduce some ambiguity, as pipelines for different refs
might include different jobs and overwrite previous statuses.

ee/app/models/project_services/github_service/status_message.rb

@@ -14,7 +14,7 @@ class GithubService
     end
 
     def context
-      "ci/gitlab/#{@ref_name}".truncate(255)
+      "ci/gitlab"
     end
 
     def description

ee/spec/models/project_services/github_service/status_message_spec.rb

@@ -87,7 +87,7 @@ describe GithubService::StatusMessage do
       end
 
       specify 'context' do
-        expect(subject.context).to eq "ci/gitlab/#{pipeline.ref}"
+        expect(subject.context).to eq "ci/gitlab"
       end
 
       context 'blocked pipeline' do