Only return mentions for existing users.

If someone changes their username on github, or if the username is
mis-typed, then the mention would come back as nil. This blows up when
we try to render a profile link.
added python edge cases to bob
Merge pull request #880 from koljakube/master

Add python edge cases for bob
Send correct params to preview endpoint. Fixes #881
Fix broken comments for completed submissions. Fixes #882
Fix seed generator
Update README to reflect migration. Fixes #883
Fix development backdoor
Update reference in comment email. Fixes #885
Update README in regards to db setup

Add db setup information for development and test environments.
Merge pull request #887 from jurglic/update-readme

Update README in regards to db setup
rust rna-transcription - rust 0.8 no longer has map_chars, use replace instead
rust - check exercises automatically with travis (rust 0.8)
rust - word-count exercise
rust anagram
add a RustCurriculum class
rust beer-song
rust point-mutations
rust - use named lifetimes in anagrams implementation (thanks @huonw)
rust - clean up beer-song