Add some support for webhooks to API; fix related bugs.

Summary:
* Standardize parameter names across API documentation and implementation: use
  'to', 'from', etc. everywhere, instead of sometimes using 'to_addr', etc.
* Add API support for creating, reading, and starting/stopping hooks.
* Concomitantly add support to the webhook service for starting/stopping hooks
  via ZeroRPC.
* Fix a bug where we wouldn't actually filter transactions on any_email.
* Simplify and standardize variable naming in the webhook service.

Test Plan: Manual testing.

Reviewers: mg

Reviewed By: mg

Differential Revision: https://review.inboxapp.com/D141