Enable deployment-notifier to post longer messages to Slack

Created by: sanderginn

Due to less frequent deploys to dotcom, the amount of PRs included in a notification can lead to a message length that exceeds the max length of 3000 characters in a text object. Posting a message to the webhook that violates this constraint fails. As a workaround, PRs are broken up into multiple sections where necessary. This does introduce a line break between sections that's unavoidable 🤷‍♂️

Also does a little polishing of the message itself.

Test plan

Tested with the Block Kit Builder by sending to myself: