Add description for beta testing a language track.  Fixes #1681
Fix language names in nitpick menu
Merge pull request #1694 from LegalizeAdulthood/beta-test-language

Add description for beta testing a language track.  Fixes #1681
Use language name, not track key. Fixes #1695. Fixes #1696.
Use team name in dropdown menu. Fixes #1697
Add documentation about track config categories. Fixes #1698
Add problem as future replacement for exercise
Tweak problem failover
Swap out exercise with problem in services
Tweak the messaging in the emails

Use the actual language name rather than the track id, as well as the name of the exercise rather than the slug.
Use track_id internally in submission
Use problem internally in user
Refer to problem rather than exercise where submission is used
Reduce usage of submission.language
Ditch exercise class
Use language name, not track id, in metadata pages
Clean up looks
Fix team name on account page
Fix avatar instructions
Add documentation for authors of new problems