Switch our media setup to use Django's staticfiles support.

Django introduced built-in support for static media files in Django 1.3,
and have been improving upon this in 1.4. This staticfiles support makes
it possible for a site to specify where to look for media files (in app
directories, specific directories, or through custom Finder classes)
and, with a 'collectstatic' command, deploy them somewhere (another
directory or some other location through a Storage module, such as
Amazon S3). It also provides built-in support for post-processing
modules.

Django Pipeline now requires this support to even operate, and Django
1.4 is very strict about STATIC_ROOT not being empty and not being equal
to MEDIA_ROOT. So it's necessary that we move along with the world,
painful as it may be.

In this new world, there's a clear distinction between STATIC_* and
MEDIA_*. STATIC_* is used for media bundled with the application (our
own CSS, JavaScript, etc.), while MEDIA_* is for user-uploaded media
(screenshots, file attachments, etc.). This means that we can actually
in theory dump the files into different places (though maybe not yet in
practice).

One advantage this all does give us is that our initial setup is
simpler. Instead of prepare-dev having to get the djblets symlinks in
the right place, we can just rely on the staticfiles stuff to pull it in
from wherever it's installed. It also means that we can simplify our
unit tests' media management.

This will require a lot of testing and will likely break in some
circumstances.

One thing that users will have to deal with when upgrading is to
update their Apache/server config file to set up an alias for
/static. Otherwise, they won't see their media files. It's an annoyance,
and one that we can't fix for them. I have plans to switch to a new
set of config files, though, which will allow us to make updates
like this without breaking users later, and so this is a good time
to force them into making changes.

Tested various Review Board pages, including the login page, dashboard,
New Review Request page, review requests, screenshots, diff viewer, and
admin UI.

Tested media with DEBUG and non-DEBUG builds, and with DEBUG but turning
on media pipelining.

Tested creating a new site and verifying that the media dirs were still
symlinking as expected into the locations specified by the user.

Tested upgrading a site and verified that the media worked as expected (after
manually updating the Apache config file to point to the static directory),
and that the old static media links were gone.

Tested that an old configuration with in-database site_media_url and
site_media_root paths were updated to create accompanying static versions.

All Unit tests passed.