Add Template model on backend and begin implementing db/fs syncing

https://assemblymade.com/buckets/wips/77

My aim here was, one, to get some eyes/discussion going on what these changes might look like, and, two, to try to slot in the DB-backed Templates model without changing too much about what's going on with the filesystem-backed templates.

TODO:
- [x] Add database fallback in `template.find()`
- [x] Make sure `POST` works as expected
- [x] Test cases

Maybe the database fallback should be added separately -- I worry about introducing too much redundant logic in the various `read` methods. And also about updating too much at once.
Fixed. I had tried that on the outermost function previously, but it looks like the inner functions needed it too.