Diff line comments resolve

What does this MR do?

Diff line comments can be resolved.

Part of #10325 (closed)

To do:

• Backend (@DouweM)
  • Fix https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5022#note_13319326. Will be made easier by https://gitlab.com/gitlab-org/gitlab-ce/issues/17237#note_13370331
  • System note when all discussions are resolved
  • Notification when all discussions are resolved
  • Write unit tests
  • Look at resolve time https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5022#note_13912743 - Fixed by 4a13aa9f
• Frontend (@iamphill)
  • Fix bugs
  • Write more feature tests
• Frontend (@connorshea)
  • Address frontend feedback
  • Feature specs for Jump feature
  • Documentation
  • Add Vue.js in a standard way