Move an individual annotation to a different group

_This is a record of a feature request sent to our support address: [#837](https://hypothesis.zendesk.com/agent/tickets/837)._

> As far as I see, by trying to edit [the annotations], there's no way to move them to a group I am a member of. Any plan to make this possible?

As a user who made a mistake and annotated in the wrong group, I want to be able to move annotations between groups, so that I don't have to waste time remaking my selections.

As a user who wants to take part in a group discussion, I want to be able to move annotations between groups, so that I can draft my annotations in one group and then move them to another for discussion.

Start with support ticket #837 and review the issue's 36-comment discussion for any decisions about permissions and group membership. Define the affected annotation and group flows before implementation; done means an individual annotation can move to a group the user belongs to without recreating its selection.

Written by the indexing model from the issue text.