(bug 41163, 44165, 44166) autocomments for claims.

This adds autocomments when claims get added or removed, or
their main snak is changed.

Caveat: When removing or changing a claim, the claim ID (GUID) is shown in the
summary. That is not very helpful. To changes this, some refactoring is
needed - this is already in the pipeline.

Change-Id: I1c836d6f54d865f83d27b5853ff0d72532c9cd6a