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.com/D19046