Add Backend Models

Currently the backend models aren't created until you attempt to send a message. I'm proposing adding a `post_syncdb` hook to inspect the settings and add the backend models.

Start by tracing where backend models are currently created when a message is sent, then inspect the settings used to select the backend. Determine how the post_syncdb hook can create the required models from those settings; done means configured backend models exist after synchronization without sending a message.

Written by the indexing model from the issue text.