Slack notifications for DEI badge level-ups

the Badging group is working on https://github.com/badging/event-diversity-and-inclusion/issues/482 and will likely need access to some kind of slack bot or app or token to make this work.

Filing this as an issue mainly so in can mark it as being blocked by #13

Start by reading the Badging group's linked issue #482 and the blocking context in #13. Determine what Slack bot, app, or token access is required for DEI badge level-up notifications; done means the needed access and integration path are agreed with the Badging group.

Written by the indexing model from the issue text.