Proposal: use the same matcher format for creating silences as for filtering alerts

[w0rm]

Currently we can filter alerts using !=, !~, =~, = matchers, but we can only create a silence with = or =~. Supporting all types of matchers in the silence form would allow us to reuse some code on the frontend.

This is especially needed for an ability to create a silence from alert (#1029). We have to stringify matchers into the location and then parse them again to populate the form. This functionality has already been implemented for filtering the alert list.

I also wonder if this would allow to reuse some code on the backend too.

[beorn7]

👍 This would go into the same direction of consolidation as the proposal 2 in #1023 .