Merging issues in Sentry should inform linked Jira tickets

### Problem Statement

1. Have Sentry issue S-A mapped to Jira J-A
2. Have Sentry issue S-B mapped to Jira J-B
3. Merge S-B into S-A.
4. Jira J-B doesn't show anything about the merge, and doesn't sync changes in any way anymore.

Resolving J-B as Done doesn't sync anything to Sentry. Resolving the Sentry issue doesn't do anything to J-B either. They're effectively entirely disconnected.

### Solution Brainstorm

Here's what I'd like to see happen when issues are merged:
1. The status for a Jira ticket attached to any issue _being merged_ into another issue should be changed to a configurable value. We'd set it to Duplicate.
2. A duplicate link should be made from the merging issue to the issue being merged with's existing ticket, if it has one.
3. A comment is posted in Jira J-B and J-A explaining what happened. J-B would get "we merged this issue into S-A" where J-A would get "we merged S-B into this issue".

At the _very least_ a comment should be posted to the J-B ticket to inform anyone watching it that it's been merged and link to the new Sentry issue.

### Product Area

Issues