Handle clientside comment adding.
Add support for POST to proxy to GitHub API.
Save new comment model to the server.

Also adds a rawBody model property so we don't try to send the
markdown-converted comment.
Merge branch 'add-comments' into dev.webcompat.com

* add-comments:
  Save new comment model to the server.
  Add support for POST to proxy to GitHub API.
  Handle clientside comment adding.