Add @mention recognition to editor

[thenanyu]

in many collaborative text editors, typing @nameofperson is semantically understood as referring to a user. for example in github @tommoor refers to the maintainer of this project.

this is really useful for meeting notes and todo lists, where you can do assignment with something like...

• [ ] action item @tommoor

and associate an item with a user

[thenanyu]

similarly, date recognition is nice to have with a command like /today that auto-populates a date

[disambiguationuk]

Pretty much the number one pain we have with outline right now

[michaelkarrer81]

This would be very much needed!

[stale[bot]]

Hey! The issue has been automatically marked as stale because it has not had recent activity. It will be closed soon if no further activity occurs. Please reply here if you wish for the issue to be kept open.

[github-actions[bot]]

Automatically closed due to inactivity

[bodhi52]

This would be very much needed!

[jlqd]

Yes, this is very much desired!

[james-freec]

one of reason to stop us use getoutline and move from notion to getoutline

[tommoor]

Sub-tasks

• [x] User search endpoint users.list with query.
• [ ] Add inline node mention_user, it will store the id and name in attributes, be a span and have atom: true set.
• [ ] Add popover triggered by @ symbol which searches through users and inserts a mention node.
• [ ] Add MentionsProcessor, similar to BacklinksProcessor that parses which mentions have been added on "documents.publish" and "documents.update" events.
• [ ] Add notification email for mentions scheduled from MentionsProcessor

[tommoor]

Closing as duplicate of #889