It toggles the 'WIP' prefix in the MR title.

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

- Return only slash commands that make sense for the current noteable
- Allow slash commands decription to be dynamic

Other improvements:

- Add permission checks in slash commands definition
- Use IssuesFinder and MergeRequestsFinder
- Use next if instead of a unless block, and use splat operator instead of flatten
Signed-off-by: NRémy Coutable <remy@rymai.me>

Improve slash command descriptions, support /due tomorrow
Signed-off-by: NRémy Coutable <remy@rymai.me>

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

Check for update_issuable permission in Notes::SlashCommandsService
Signed-off-by: NRémy Coutable <remy@rymai.me>

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: NRémy Coutable <remy@rymai.me>