Set up CASEI MI Deployment Slack Bot

[heidimok]

Is your feature request related to a problem? Please describe. This isn't based on an immediate problem, it's communications/experience improvement. We recently added automated deployments for the MI so that the process is less manual. The value of a Slackbot message is that non-developers can know when the MI is deployed, removing the need for developers to inform the channel each time.

Describe the solution you'd like Similar to the CASEI UI where a deployment is announced in the ADMG Slack tread

Steps

• [x] Create a slack app on the IMPACT workspace and get a token for it
• [ ] Add the token to each github environment's secrets

Other notes. These were paths Anthony L had suggested:

• Create new webhook for ADMG backend (link) and copy the CASEI notification strategy (link). This will be the quickest but is a legacy technique.
• Create a new Slack App for ADMG backend (link) and use the notification strategy currently in the ADMG backend repo (link). This will be slower but is more proper.

[heidimok]

Teamwork: @EJwalker13 to help with the slack app set up + token, then pass on to @Tammo-Feldmann for step 2.