Added support for DELETE request to api/session and created test cases for it.

The API currently creates sessions (with associated cookies), but there's
currently no way through the API to log out. This makes it so that a script can
authenticate, do it's job, and then remove the session afterwards.

Testing Done: Ran unit tests.

Bugs closed: 3294

Based on Peter Tran's work reviewed at https://reviews.reviewboard.org/r/5817/