Filter muted submissions when suggesting work

Adds a unmuted_for(username) class method to submission with test and
chains that method in user to prevent muted suggestions from being mixed
in when random submissions are suggested. Fixes #747
Fix bug in db:seed
Improve seed comment API
Clean up method API on User
Add completed exercises to user in db:seed
Never return an array of languages from User#random_work
Refactor seed generator
Move auth helpers into auth controller
Delete unused helper method
Only nitpick completed exercises

This reverts the experiment where people could go nitpick the exercise
they're currently on.
Fix buggy completed list in API
Remove unused code
Use :github and :tag and keys for gems on github
Move newrelic_rpm into production group in Gemfile
Merge pull request #764 from iHiD/tidy_gemfile

Use :gitub key, :tag key, and production group in Gemfile
ruby saddle-points fix coordinates #744
javascript saddle-points fix coordinates #744
Merge pull request #765 from etrepum/saddle-points-744

Saddle points 744
Put newrelic back in both groups
Don't use a string when you can use a symbol