Pass through template name for the configure_extension view.

djblets' configure_extension allows specifying a custom template, but review
board's didn't. This meant that if an extension wanted something special on its
settings page, it would have to re-implement the code from
reviewboard.extensions.views.
Make crypto_utils backwards compatible with python-crypto on CentOS 6.4

CentOS ships an older version of pycrypto by default, with the newer version
only available in EPEL. This change makes Review Board work with either.

Testing done: Was able to get ReviewBoard to work on CentOS 6.4

Reviewed at https://reviews.reviewboard.org/r/4769/
Fixes bug 3110