Comment mentions

[ssddanbrown]

Related to #560.

Todo

• [ ] Finish off editor UI.
• [ ] Address remaining todos in code changes
• [ ] Add lookup for users
  • [ ] Limit to those with ability to create/update comments for now.
• [ ] Back-end comment parsing
• [ ] Notification on mention
• [ ] Notification preferences - Add option for comment mentions.
• [ ] Test cases for back-end functionality.

Considerations

• How do we handle mention changes on comment updates.
• Do we need rate limiting to prevent spam like page update notifications.
• Are these useful if we don't opt-in users to them by default?

Doc Updates

• Update advisory - Anyone with comment create/update will technically be able to access a list of system users.