Merging issues - allow user to choose "main issue"

### Problem Statement

When I merge issues, often one of the issues is "the main issue", and the others are just side-effects.
Common example:
* service A calls service B which calls service C
* service C has an exception
* all 3 services log an Error
* 3 issues are created (let's call them A,B, and C), but I merge them to 1 issue
* The issue for service C is "the main issue" - it is the root cause, and has the most valuable information. the other issues just say something like "call to service C failed"

So when I merge the issues, I wish issue C will be the picked as the main issue, so the issue has a meaningful title and the example event displayed will be from issue C.
But Sentry chooses an issue arbitrarily (maybe the first one?), and there's no way to change that.

### Solution Brainstorm

I wish the UI offered a way of selecting "the main issue" when merging.

I had something like this in mind, please forgive my poor graphic skills:
(the orange icon on the left, which I've picked from google-images, marks the issue selected to be "main")
(of course the UI is not my concern, this is just a suggestion)

It would also be nice to be able to change that later in the "Merged Issues" tab,
but that's not a must - we can just unmerge and merge again.