Strip off #index_header before starting the diff router.

Backbone's router will turn hashes into URL components. This is a problem for
the diff page, which uses #index_header to jump past the review request
details. I've added some code to strip out the hash before we start Backbone's
history tracking, which fixes the URL.

Testing done:
Loaded /diff/#index_header and saw that the URL changed to /diff/# instead of
/diff/index_header.

Reviewed at https://reviews.reviewboard.org/r/4701/