These changes enable Review Board to use x.509 certificates to be used alongside the builtin username/password authentication backend. This functionality will be useful for deployments in environments with a Public Key Infrastructure set up.

I deliberately avoided modifying post-review to support client certificates due to a lack of decent support for them in Python's high-level libraries. Instead, I modified the user preferences view to permit authenticated users to set their Django password. Django's built-in support for multiple authentication backends will then permit them to use the builtin auth backend with post-review.

This diff was originally reviewed at http://reviews.review-board.org/r/938/

Tested on my test Review Board server, running Apache/mod_python,mod_ssl/Django 1.1.