enable annotations to be posted to multiple groups

## Feature Request Form
From a user: One thing I really like is that you can move annotations from one group into another (or into public). But it would be even better, if annotations could be shared with multiple groups. Because if I move the note from group A to group B, users who are a member of group A but not of group B will no longer see the annotation.

Start with the feature request and map how moving an annotation between groups currently affects visibility. Define the behavior for annotations shared with multiple groups, including public visibility, and confirm that members of each selected group can still see the annotation without removing access for other groups.

Written by the indexing model from the issue text.