@mentions in comments

[nickisnoble]

• [ ] Comments use markdown (preferable) or action_text
• [ ] This change is non-destructive to existing comments
• [ ] Users can be @mentioned by handle
  • [ ] if #7 is not yet complete, mentioned users should just hyperlink their profile?

[nickisnoble]

Might be a fun implementation: https://editor.nicholaschen.io/