Allow comment log per silence

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.

Start by tracing how silence objects are created, stored, and addressed by ID in the Alertmanager codebase. Define the behavior for requiring an initial comment, appending immutable author/timestamp/comment entries, and preserving existing silences; done means the API and persisted silence representation support the proposed comment-log semantics.