Extract code into IssueBoardEntity / serializer (follow up)

Clear up some technical debt by extracting code from boards/issues_controller.rb into a serializer.

The corresponding EE version of this MR is at https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8679

The original version of this MR (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22760) was rolled back due to issues in EE.