Extract EE specific files/lines for app/models/concerns (discussion)

We have the following files containing EE specific code. We should move them to ee/

• app/models/concerns/issuable.rb
• app/models/concerns/mentionable.rb
• app/models/concerns/mentionable/reference_regexes.rb
• app/models/concerns/protected_ref_access.rb

Should be simple to do. Edited: Sorry, not simple at all!