Highlight current user @mentions differently

Problem

Currently it is hard to find mentions of yourself in long discussion threads/comments

Design

• Needs https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20509 to be merged!
• Highlight current user @mentions differently (should work in discussions and anywhere else https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20509 has an effect)
• Colors are background-color:orange-100
• Implementation should be done on the frontend