Add ability to update group admin

[ryanchristo]

In the current implementation, the admin of a group is unable to update the group admin.

As a user, I should have the ability to create a group with myself as an admin and then transition to a group with policy as admin at a later point in time. I should be able to input an address, so that I could also change the admin to another user account.

This may need to be separate from the "edit group" page and similar to adding members on the group details page, meaning we may want to add an "edit admin" action next to the admin listed on the page similar to how we have an "edit members" action.