@ mention on task lists in issues/MRs to create todos

Summary

We support * [ ] syntax for adding checklists to issue and MR descriptions. This is awesome. Let's go one step further and let people add @ mentions to the end of the checklist items and then assign todos to those people for the specific item. Then when a checklist is marked completed, automatically close the todo for the person or vice versa.

Relevant links, prior art

Dropbox Paper does this well.