Allow group owners to transfer ownership

[mbramson]

Currently there is no way for an owner of a group to give ownership to another user. Implement this.

If the current user is the owner of a group, when they are editing the group, they should be able to transfer ownership of the group. This updates the group with a new owner.

Users with the group roles (not yet implemented) sufficient to edit a group, should not be allowed to transfer ownership of the group (or see the UI for this).

It will likely make sense to add a validation step to the group changeset, that ensures that any user changing the owner of a group is either the current owner of the group or an admin.