Allow comment log per silence

[fabxc]

For long running silences it is feasible to have a log of comments rather than a single one to easily add information on the current status of an underlying problem.
This can be done within a single comment, but usually comments are fairly short and people might want to clarify. For that the silence creator is not necessarily the best reference.

I propose silence objects to have a list of <author, timestamp, comment>. Generally that will be so short that we can just store it within the same blob.

Semantics should be that you can add a comment to a silence by ID and have to provide an initial comment when creating the silence. Existing comments cannot be modified.

[stuartnelson3]

Currently, silences require a comment for creation. The internal model for silences, however, is that any modification creates a new silence.

Having an immutable comment list, where clicking "update" in UI requires and appends a new comment/author/timestamp to the silence's comment log, makes sense to me. If someone has to extend/modify a silence, having the context would be helpful. It would also provide continuity between a silence and its updated version, where currently there is none.

Thoughts?
@brian-brazil @brancz @fabxc @beorn7 @grobie

[brian-brazil]

I think this is getting into incident management rather than alert management. If someone wants to add an additional comment they can alter the existing silence comment, but beyond that this probably belongs in an issue tracker.

[beorn7]

For me, this would be "nice to have" at best. Let's only think about it once we have got the much more pressing issues under control.