[review together] Allow the user to specify their own hub in the extension config.

With TogetherJS, all communication is done via WebRTC, but people may not be
pleased having the negotiation happen via Mozilla's public hub. This change
adds configuration to allow people to specify their own hub.

Testing Done:
A hub server was run locally. Its URL was used in the extension configuration.
The local server was used successfully to support the extension. To test the
hub used is actually the one indicated, tried to launch the chat app while the
local server is not on. The chat would still launch but using the "add friend"
link on another browser could not connect the two clients.

To test the default hub is used when no hub is specified, remove any input in
the extension configuration screen. Launch the chat app and using another
browser connect to the first client with the "Add friend" link. The clients
should be able to see each other's mouse movements.

Reviewed at https://reviews.reviewboard.org/r/6102/
[rbxmlreview] Fix up pyflakes warnings

Trivial change to fix some warnings.
[shipit_ascii_art] Fix up pyflakes warnings.

Trivial change to fix some warnings in shipit_ascii_art.
[rbslack] Don't cause a 500 if notification fails.

In the case where something fails when trying to send to the slack.com webhook,
we shouldn't show the user an error 500. This change catches all exceptions and
logs them.

Reviewed at https://reviews.reviewboard.org/r/6354/
[rbdemo] Add some checks to prevent us from breaking the demo site.

This adds some pre-reset checks to rbdemo's reset-demo command that
ensures we have all the proper permissions and abilities we need prior
to resetting the demo database state. This includes read-access to the
fixtures and media directories, write access to the destination media
directory, and the ability to change ownership. It also validates that
the specified user and group are valid.

Reviewed at https://reviews.reviewboard.org/r/6338/
Improve defaults and metadata for rbslack.

This adds a better name, description and URL for rbslack for the
extensions page, along with bumping up the development status to Beta.

It also removes the default "#notifications" channel, allowing the
configured channel on the Incoming Webhook to take precedence unless
specificially overridden.

Reviewed at https://reviews.reviewboard.org/r/6569/