WIP: Add remind_me quick action

What does this MR do?

Adds the "remind_me" quick action, which allows users to specify a future time for a "todo" to be created on an issue or merge request.

The format for the command is \remind_me 1w 2d 3h 4m 5s. If a todo currently exists, "remind_me" marks it as done, then recreates the todo at the specified time.

Are there points in the code the reviewer needs to double check?

I created a function in TodoService (mark_todo_from_reminder) which creates a todo from a user_id rather than current_user. I think redis is trustworthy, but it's worth double-checking.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered

What are the relevant issue numbers?

#41339 (moved)