Add line-level comments on changesets

This builds on and completes @pepegar's prior work to implement line-level comments on changesets.

Overview:
* Fixed errors when trying to add comments on changeset as a whole or reply to line comments
* Fixed mouseover formatting issues
* Mouseover is now active for the whole row, and highlights the line just the same as on a file diff
* Useful display of filename/path/changeset info in the comment dialog title
* Useful display of comment path info in the Code Comments table
* Views are as agnostic as possible about what elements they are attached to, contents of TH cells, etc.
* Use a single template for line comments (view passes an argument to tell it how many columns to take up)
* CSS/code style/bug fixes

Known issues:
* Comment permalinks are currently only working on changeset view because I added some code with a setTimeout.  The comments are getting added by Backbone, so if we include anchors for each comment, they don't exist until the Backbone views have been rendered.  Even hooking this in to _.defer() doesn't make it wait long enough.  I need to sleep on this one, but suggestions would be welcome!