Add silence logging option

[anarcat]

Past silences are really important to postmortem our alert management operations. We need to get "when" and "what kind of" create/edit/delete silences. So this PR added optional function that logging silence change.

Reroll of #2536 to address review item.

[SuperQ]

Needs a rebase.

[anarcat]

i rebased, but the history is not super clean, should i squash?

[TheMeier]

Squashing the commit sounds like a good idea to me. Just a question would it make sense to add a minimal test for the new function?

[anarcat]

probably! not sure where to start with that though

[TheMeier]

How about something like this https://github.com/TheMeier/alertmanager/commit/ada4ea8dc92d4464810ab23d80b09ab69f29ccfd

[anarcat]

How about something like this TheMeier@ada4ea8

that looks fantastic.

[TheMeier]

So how about you steal it into your branch and then squash everything ;)

[anarcat]

So how about you steal it into your branch and then squash everything ;)

done!

... and rebased.