Cache user login & register pages.

Looking at the various varnish stats, this will hopefully improve the
cache hit ratio.  The user login page was also previously setting a test
session cookie -- likely to ensure the browser supports cookies.  We now
remove that test cookie, as it would invalidate the cache.

Conflicts:
	localwiki/templates/registration/login.html
	localwiki/templates/registration/registration_form.html
	localwiki/users/urls.py
	localwiki/users/views.py