Add webhook management API

Review Request #7483 — Created July 3, 2015 and submitted — Latest diff uploaded

Information

Review Board
release-2.5.x
0db4571...

Reviewers

Webhooks are now able to be created, retrieved, updated, or deleted via
the API. All webhook APIs are limited to superusers and local site
admins (for local site webhooks). Unit tests have been added to test
the basic CRUD functionality of the resource.

Ran unit tests.