Simplify logout view.

Instead of using django.contrib.auth.views.logout with a next_page of our login
URL, use django.contrib.auth.views.logout_then_login, which does the same
thing. Additionally, move the URLspec from reviewboard/urls.py to
reviewboard/accounts/urls.py.

Testing done:
* Ran unit tests
* Logged out of my devserver and saw that I was sucessfully redirected to the
  login page.

Reviewed at https://reviews.reviewboard.org/r/4930/