Now that we have support for webhooks in Review Board, it's time to make

them completely awesome and flexible, without being annoyingly hard to

use.
Previously, our webhooks were global across repositories and were forced

into pre-built JSON payloads. This is pretty par-for-the-course for

webhook implementations, but a few implementations out there had some

nice features that were worth bringing in.
Now, webhooks can be tied to one or more repositories, can be set to

apply only for events not matching a repository (such as review requests

that are only used for file uploads), or can be set to always fire

regardless of whether there's a repository.
They can be set to output JSON, XML, or form data payloads (in which

case there will be a payload=<encoded form data> sent to the server.
Payloads are also no longer limited to what we generate. Users can

toggle an option for specifying a custom payload, which gives them a

CodeMirror widget they can edit. This supports the Django template

format, allowing the use of template tags, with the original payload

provided as the variable context. Several scary tags (such as include,

load, etc.) are disabled, and RequestContext is not used, keeping this

safe.
The webhook UI has been updated to provide a bit more guidance, especially

with the new options.
There were a few changes under the hood as well. Instead of a

X-ReviewBoard-Signature, we now use the more standard X-Hub-Signature

(followed by GitHub and others). We also provide a proper User-Agent.

Added unit tests for all the conditions.
Played around with creating some webhooks.
Database evolved successfully.