PROCESS CHANGE: Add a rotation app to slack to remind people when they are a community contact

[grantr]

We now have a community contact rotation for eventing and other WGs, but there's no great way to notify people when they're up next in the rotation. This seems like a good case for a slack app to manage the schedule and notifications.

Bonus, this might be useful for the TOC presentation rotation.

Here are a few blogs about such things I found in a quick google search: https://medium.com/in-the-weeds/round-robin-rotating-responsibilities-responsibly-f4a9b44e174c https://dev.to/kimmaida/hello-rota-a-slackbot-for-rotation-management-216c