Create a Hypothesis Staff admin tool to allow us to add owners/admin to groups

[mkdir-washington-edu]

Slack conversation: https://hypothes-is.slack.com/archives/C4K6M7P5E/p1734540521176849

The problem - Hypothesis staff will need to occasionally assign someone to be an owner or an admin of a group where none of us have in-group permissions to do so.

Solution - build an admin pages tool to allow us to add new owners and admins to a group.

[robertknight]

I left a note in Slack that there are closely related problems with the existing group admin UI that we need to discuss.