Bump dependency to Django 1.5.x

Pull in Django 1.5.4 and Djblets 0.8.
Add quotes to {% url %} tags.

The url templatetag has been changed to require quotes around string arguments
(like everything else does). Update our templates accordingly.

Testing done:
* Ran unit tests.
* Grepped for "url" in all our templates.

Reviewed at https://reviews.reviewboard.org/r/4619/
Update for PUT payload changes in the test client.

In 1.5, django.test.Client changed the behavior of PUT, OPTIONS, and DELETE. In
1.4.x and prior, OPTIONS and DELETE behaved like GET (encoded any data in the
query string), and PUT behaved like POST (encoding data as multipart). These
are now very generic, and it's up to us to manually encode the data and set the
content-type for PUT requests if we want it to get to the server.

Testing done: Ran unit tests.
Reviewed at https://reviews.reviewboard.org/r/4620/
Fix the subject in test_put_reviewrequestdraft_publish

In Django 1.5.x, the locmem email backend will trigger header validation and
generate a message ID. This means that the subject when publishing a second
draft will be "Re: ...". Our previous implementation of the assertion was
actually incorrect, but it took a while to convince myself of that.

Testing done: Ran unit tests.
Reviewed at http://reviews.reviewboard.org/r/4621/
Bump python requirement to 2.6.x