Creating a filter with the Mastodon API doesn't assign filter_id

When you create a filter by POSTing to /api/v1/filters, its filter_id is left as NULL/nil. Among other things, this prevents SubwayTooter from being able to delete filters.

Suggested fix: either remove the filter_id field from Filter and use id instead, or find some way to make sure filter_id always gets set. The former is probably the better option though it will require checking that a user isn't trying to delete someone else's filters. The latter isn't great because then you have two candidate keys: id and (user_id, filter_id), but it would be consistent with the way Mastodon treats filter ids.