Add JavaScript for validating diff files.

This is the client side of the diff validation resource. To use this, one
instantiates the model, sets the repository/diff/parent diff/basedir, and calls
save(). Either the "success" or "error" callbacks in the options object passed
to save will be called, based on whether the diff parses correctly and the
source files exist.

Testing done:
- Ran unit tests.
- Used this with a number of different diffs that caused various errors.

Reviewed at https://reviews.reviewboard.org/r/4320/
Improve rendering of new files in the diff viewer.

New files no longer have the blank line number and code columns for the
left-hand side of the diff. It just has the new code's columns.