Diff line comments resolve

## What does this MR do?

Diff line comments can be resolved.

Part of #10325 

To do:

- [x] Backend (@DouweM)
  - [x] 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
  - [x] System note when all discussions are resolved
  - [x] Notification when all discussions are resolved
  - [x] Write unit tests
  - [x] Look at resolve time https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5022#note_13912743 - Fixed by 4a13aa9
- [x] Frontend (@iamphill)
  - [x] Fix bugs
  - [x] Write more feature tests 
- [x] Frontend (@connorshea)
  - [x] Address frontend feedback
  - [x] Feature specs for Jump feature
  - [x] Documentation
  - [x] Add Vue.js in a standard way

See merge request !5022

Some important things to note:

- commands are removed from noteable.description / note.note
- commands are translated to params so that they are treated as normal
  params in noteable Creation services
- the logic is not in the models but in the Creation services, which is
  the right place for advanced logic that has nothing to do with what
  models should be responsible of!
- UI/JS needs to be updated to handle notes which consist of commands
  only
- the `/merge` command is not handled yet

Other improvements:

- Don't process commands in commit notes and display a flash is note is only commands
- Add autocomplete for slash commands
- Add description and params to slash command DSL methods
- Ensure replying by email with a commands-only note works
- Use :subscription_event instead of calling noteable.subscribe
- Support :todo_event in IssuableBaseService

Signed-off-by: Rémy Coutable <remy@rymai.me>