[SCHEMA] Consolidate Webhook filters with new `Lens` class.

Summary:
- Combines `DatabaseFilter` and `TransactionalDataFilter` into the single `Lens` object which is used to filter data in both cases.

- Called "lens" because it is used to filter/focus something. Also because the word "filter" is used way too much in SQLalchemy and it would be confusing to differentiate.

- Still left to do is connect this to webhooks as filter objects and write moar tests.

Test Plan: Run migration and all tests pass

Reviewers: emfree

Reviewed By: emfree

Differential Revision: https://review.inboxapp.com/D135