Settings rejigger + S3 for media storage

There are now four settings files: base, dev, production, and production-admin.
* `base` is shared by the other three; it includes only apps needed by all servers
* `production` now includes environmental configurations. This settings file will be used on the normal web servers
* `production-admin` includes production, but also includes apps for the admin screens (including sessions, messages, users, etc. etc.). It's also now configured to upload images to S3. This app would be used by editors, and its access should be limited.
* `dev` includes all apps, but serves everything from the local disk

Technically, there's also `test`, which may just be replaced with `dev` at some point.

The URLs file now pays attention to which apps are installed, no longer including admin nor tinymce in the production. This does not account for the `paygov` app, which may live on the admin servers or completely separately.

@annalee @khandelwal @seanherron - Would you mind reviewing this setup?