R1:dc2995c4ca3f

Refine core webhook implementation somewhat

Summary:
Depends on D19045. Ref T11330.

- View/regenerate HMAC keys.
- Pretty JSON.
- Readable status transactions.
- test, silent, secure flags.
- Dates on request view.
- More icons.
- Can test any object.
- GC for requests.

Test Plan: Went through each feature poking at it in the web UI and with `bin/webhook call ...` / `bin/garbage collect ...`.

Subscribers: ftdysa

Maniphest Tasks: T11330

Differential Revision: https://secure.phabricator.