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:
- [ ] Add database fallback in `template.find()`
- [ ] Make sure `POST` works as expected
- [ ] Test cases