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.
Add redirects from the old dashboard
Fix dashboard links.
Fix nitpick navigation
Merge pull request #702 from Gaelan/patch-1

Fix dashboard links.
updating bob test error code help test to be more accurate
Merge pull request #703 from koriroys/kjr/ruby-bob-test

updating bob test error code help test to be more accurate
haskell robot-simulator cleanup (tests imported untested API surface)
Merge pull request #705 from etrepum/haskell-robot-fix

haskell robot-simulator cleanup (tests imported untested API surface)
rewrite simple-linked-list README
ruby simple-linked-list
haskell simple-linked-list
fix simple-linked-list.md formatting
Update user-facing notification

The command to update a gem is 'gem update', not 'gem upgrade'.
Asking people to upgrade their gem is misleading (I know because
the first thing I typed was 'gem upgrade exercism').
rewrite linked-list README
javascript linked-list
ruby linked-list
Merge pull request #706 from Trevoke/patch-1

Update user-facing notification
another one bites the dust.