Resolve 3 "Extract EE specific files/lines for app/views/shared/issuable"

What does this MR do?

Unify:

• app/views/shared/issuable/form/_metadata.html.haml
• app/views/shared/issuable/_sidebar_assignees.html.haml (Only CE needs changes)

Are there points in the code the reviewer needs to double check?

CE at: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/19777

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer

What are the relevant issue numbers?

Part of #6145 (closed)