Add initial migration (missing from repo)

The initial migration was missing - and undetected as the initial
syncdb does the job.
Refactor use of py-trello internally

The internal _fetch, _pull, _push methods, which were a bit clunky, have
been replaced by _update_remote, _create_remote and _delete_remote
which use the TrelloClient directly rather than trying to shoehorn in
use of the py-trell WebHook model, which doesn't support a save
operation.

This makes the integration cleaner (as we can now save as well as
create and delete), and reduces our use of the py-trello app to just
the core API request-response.

Also includes a migration to add an is_active flag to the Webhook model.