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.

Saw the new information in the extensions list.
Posted without specifying a channel in the extension settings, and saw

it use the corret channel specified in the Incoming Webhook settings.