Clean up Merge Request Widget Entity

As discussed in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6181#note_82561936 the MR widget entity need to be cleaned up and refactored.

• remove duplicate properties (probably a bad merge conflict resolution)
• extract the current_user object into a dedicated entity so that it can be extended in EE
• regroup all user permissions under current_user object (either from CE or EE)