### What
This PR changes the slack-app addon to use slack-api's scheduleMessage
instead of postMessage.
### Why
When using postMessage we had to find the channel id in order to be able
to post the message to the channel. scheduleMessage allows using the
channel name instead of the id, which saves the entire struggle of
finding the channel name. It did mean that we had to move to defining
blocks of content instead of the easier formatting we did with
postMessage.
### Message look
4ad370450d