Show similar existing silences when creating a new silence

It would be useful if, when creating a new silence, a list of similar silences was shown. My use case is for avoiding accidental silencing of similar but separate alerts mistakenly. I recently mistakenly silenced an alert that eventually lead to an outage because the alert name was the same, and another label (k8s cluster + namespace) was so similar that I assumed this second alert that I now was creating a silence for should have fallen under the first silence, and that the original silence had been created without an extended duration (default 2h).

If while I was creating that second silence, I could have seen something like "hey, there's already silences for this alertname but with these other label values" this could have been avoided.

cc @pracucci

No files, tests, or entry points are named. Begin by locating the silence-creation flow and existing silence matching, then define which labels make silences similar and how differing values should be presented. Verify the behavior prevents accidental overlap without changing silence creation itself.