Fixing getLatestMembers performance when limiting by 1

Uses a more efficient aggregate query, to avoid the database having to
do a filesort on a temporary table because of joining and sorting.

On large Member tables, this reduces the querying from ~400ms in one
to around ~60ms in two (one to get the max ID, and another to fetch
the Member record by ID)

Also removed the administrators group from the query. I don't see why
you'd want to see the latest administrator join the site.