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 CE version of this MR is at https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23511
The original version of this MR (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22760) was rolled back due to issues in EE.
Edited by Coung Ngo