Add the notion of a mention.

- Look through the comment to find @username
- Add the mention list at the end of the nitpick with links to the exercism profile
- Add that to the list of submission participants
  - that taps into the existing notification system
  - exploit the fact that no guard exists to keep someone out of a submission given the correct submission ID
- Tested all the things that I could find. I couldn't figure out how to test sending emails. I didn't see emails sent for any nits.
Merge pull request #700 from barrettclark/issue284_mentions

Add the notion of a mention.