Taggable notes and sources

Use either [mongoid_taggable_with_context](https://github.com/lgs/mongoid_taggable_with_context) or a custom concern.

- [x] Model integration
- [x] Model specs
- [x] Add to Mutations
- [x] Fix Mutations specs
- [ ] Add UI
- [ ] Fake filter UI on `NotesController#index`