Add an extension for notifying Slack.com

Slack.com is a popular business chat service which targets software developers.
One of its neat features is that it integrates with a lot of third-party
services, allowing them to send notifications.

This change adds a new extension which integrates Review Board with Slack,
using their "incoming webhooks" integration. This will notify for all of the
usual events, including links to the affected review requests.

Testing done:
Configured this extension in my local devserver and ran through the motions.

Reviewed at https://reviews.reviewboard.org/r/6024/
Add an extension for a "Note to Reviewers" field on review requests.

This extension adds a "Note to Reviewers" field below "Testing Done,"
which developers can use to give instructions on what to focus on or
to pre-answer questions that may be asked, without lumping those into
the description of the change.

This field is saved and published along with the draft.

While useful, it largely functions as a demo for how to add a simple
field.

Reviewed at https://reviews.reviewboard.org/r/6026/