Make update script call into setup (which detects it should do an update)

Fixes #962, #999, #1035

To test:
* Pull this code
* Add some videos through the video update page.
* Run `./manage.py setup` and do a fresh setup (say yes, when it asks you if you want to delete your database)
* Run `./manage.py katest unregister` to delete all registration info (log in through website as admin to verify it asks for registration information)
* Run `./manage.py update`
* Check that:
    * No errors
    * When you load the server as admin, it doesn't ask for registration info
    * All videos show as available in the webpages.

@wangguan59 could you try this out?  A little complicated, but we did this kind of testing together back in the "install_wizard" days :)