Add a new reactive action to allow labelling issuables

This adds a new reactive action to allow labelling issuables.

Usage

@gitlab-bot label ~"group::source code"

Limitations

• Commands work on new issue/MR description as well as in notes.
• Multiple commands can be sent (commands must be at the beginning of lines).
• Only community members will be able to request labelling.
• Only the author of the noteable will be able to request labelling.
• Only a group label can be requested for now.
• Rate limiting is set to 60 label commands per author per hour.

Security checks

• Are new labels created automatically when the /label command sets non-existing labels?
  • No, the command only sets labels that already exist: https://gitlab.com/gitlab-org/gitlab/-/blob/78397f0f0f00479da05d719b64d07772881b09e3/app/services/quick_actions/interpret_service.rb#L104
• Implement a simple rate limiting per author (we can leverage Triage.cache for that)
  • Rate limiting is set to 60 label commands per author per hour

Post-merge task

I don't plan to specifically communicate this change as this will be advertised in the "Thank you for your contribution message" as a follow-up MR once we make sure it works as expected in production.

• Make sure it works as expected in production (e.g. with @rymai-forks): !44 (comment 484800458)
• Advertise in the "Thank you for your contribution message" as a follow-up MR: #70 (closed)

Closes #63.