Show similar existing silences when creating a new silence

[cstyan]

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

[beorn7]

Also, similar recently expired silences would be good to show to make it easier to recreate silences.