Move some stuff out of contributors/app/services/gitlab_rest/issues.rb

The event, rewards, and content constants and methods should probably be in the event/reward/content services themselves?

The following discussion from !1630 (merged) should be addressed:

• @leetickett-gitlab started a discussion: (+1 comment)

  In a follow-up we should move these out into event/content specific classes I think (the services which likely already exist)