Edit comments

Fixes #184.

@ZachGersh and I enabled editing comments and nitpicks.  If the user is the author of any comment or nitpick on a submission page, they'll see a clickable pencil icon, which takes them to a separate edit view.

Improvements:
- There's duplication between the edit_nit and edit_comment views - could they be collapsed into one view?
- Discard the view altogether and replace it with HTML5 editing in place/jQuery.