Wraps deployment notifications in Slack threads
What does this MR do?
When deployer sends the initial notification, a slack message will be posted, and the slack message ID ('ts') will be stored in a temp file. Upcoming notifications will read from this temp file and send a message as a thread.
This change is under a feature flag that is currently disabled.
Related to gitlab-com/gl-infra/delivery#1372 (closed)
Edited by Mayra Cabrera