Implement simple message annotations

Implement simple message annotations

This feature makes it possible to "pull in" a message in a thread and highlight
it with an annotation (free text format). This will list the message in a table
along with the annotation and who made it.

Annotations have to be attached to a specific message - for a "generic" one it
makes sense to attach it to the latest message available, as that will put it
at the correct place in time.