Allow sorting by MAU

[Nothing4You]

You can sort instances by users but only total users, not MAU.
I'm pretty sure this worked before the redesign, as I used that from time to time.
For many purposes MAU are a much more useful metric.

[Nothing4You]

turns out this is already possible, but only in some places.

on https://fedidb.com/servers you can sort by MAU, on https://fedidb.com/software/mastodon you can't.