Comments

[hmil]

Addresses #73

Progress:

• [x] comment a review
• [x] comment a comment
• [x] upvote/downvote
• [x] localize strings
• [x] edit a comment
• [x] delete a comment
• [x] analytics
• [x] email notification
• [x] popup for non connected users
• [x] warning notice when commenting on own anonymous review
• [x] test failure scenarios (eg. comment too long)

[hmil]

TODO:

• stress test
• refactor "Comment" model with "review" and "parent" properties + document this model.