Handle Issue States for Un-Merged Issues

## Objective:

Code where we handle un-merge: https://github.com/getsentry/sentry/blob/master/src/sentry/tasks/unmerge.py#L465

Snuba records which Issues are merged. The primary Issue is the one that has the most events. The other Issues will get merged into the primary Issue.

We should delete the merged Issue's EscalatingForecasts and create a new forecast for the separated Issues. On un-merge, we should assign the GroupStatus and GroupInboxReason of the merged Issue to the separated Issues.

The primary Issue should inherit the current GroupHistory of the original merged Issue. No new records need to be added to this Issue's GroupHistory.

The child Issues that were split out should have a new entry in their GroupHistory with the inherited GroupStatus.