Allow users to add or remove themselves to/from groups using the API.

Previously, there was no way for a user to add themselves to a group, or
remove themselves from a group. Only admins were able to do this. There
was also a bad query preventing adding on non-localsites.

Now a user can add themselves to any group they'd have access to, and
remove themselves from any group they're on. This works with both global
groups and groups on Local Sites.
Support adding/removing users with RB.ReviewGroup.

RB.ReviewGroup can now add users to the group or remove users from the
group through the new addUser/removeUser functions.

This works much in the same way as watched object tracking. We have a
proxy object, GroupMember, that we can call save() or destroy() on, and
it will invoke the POST or DELETE request with the data needed for
manipulating that membership.
Add a darker search icon to the spritesheet.

This adds a darker version of the search icon, intended for lighter
backgrounds. It's otherwise of the same style.

For consistency, I placed this as the next icon after the lighter search
icon, shifting over some other icons.

I also shifted the light icon just a bit to better pixel-align it.
Run initialize() in test setUp() methods.

In Django 1.6, the test Client.login() method no longer uses a request, which
means that our middleware that loads the siteconfig isn't guaranteed to run.
For one of our tests, it was hitting the case that the user would be
authenticated against ModelBackend, and then when we tried to do an actual
request, it would load the siteconfig, replacing
settings.AUTHENTICATION_BACKENDS. Once this happened, it would check that
user.backend was in the loaded backends, and when it wasn't, set request.user
to be AnonymousUser.

I've solved this issue by calling initialize() inside TestCase.setUp(). A lot
of our TestCase subclasses weren't calling the super class' setUp(), so I've
fixed that.

This change isn't required on Django 1.5, but it is backwards compatible. I'm
submitting it now so we can get it in and reduce the size of the Django 1.6
patch.

Testing done: Ran unit tests.

Reviewed at https://reviews.reviewboard.org/r/5312/