Add a count of the group's members to the `/groups/{pubid}/edit/members` page

Depends on https://github.com/hypothesis/h/issues/9133.

Add a count of the group's number of members to the `/groups/{pubid}/edit` page.

This can be seen in the designs here: https://www.figma.com/design/jon1U01LGSLcx7PWtZ9TPZ/Hypothesis---Group-Management?node-id=2677-127&node-type=canvas&t=WbQZqgZOTMUcskEI-0

Note: since the read-group-members API's responses are going to be paginated (see https://github.com/hypothesis/h/issues/9133) this will need to be implemented by adding the total count to the paginated read-group-members API's responses and having the frontend read it from there.