Ability to select groups from adder

Nobody has claimed this yet.

From Zendesk ticket: https://hypothesis.zendesk.com/agent/tickets/2190
### Problem you are trying to address with this feature
This user separates annotations into two private groups ("Things I don't understand" and "Things to ponder"). They would like to be able to select a group from the adder so they can easily switch between groups while annotating a single page/document.

This user's case could be helped by using tags rather than groups to separate out their annotations, but the user would not have an activity page for both of those groups.

Also, the ability to select a group could help clear up confusion for education users, who often find that students annotate in the "Public" layer rather than the designated group for the class.

Start with the issue description and the linked Zendesk ticket, then locate the annotation adder and the existing group-selection flow in the product. Clarify how users choose between private groups and the Public layer while annotating a page, and identify the relevant interaction and acceptance criteria before implementation.

Written by the indexing model from the issue text.