create smart slack messages

[marcelkornblum]

The following event should generate custom messages* in the first instance

• pull request opened
• pull request updated
• pull request closed
• deployment to envs: test, staging, production

Each of those events may not make a new message; ideally the messages are around the point in the workflow and combine all the details (PR status, deployment status, etc) into a single message that is updated on Slack as its status changes.

We should also @ slack users as appropriate as we have the mappings from GitHub usernames available.

This is all in order to avoid the wall-of-text we always get.