fix: use faster method to fetch user count

fix: use faster method to fetch user count

countUsers: the actual user count reported by the backend.
countSeenUsers: count every user who was logged in once.

1: We should avoid expensive operations (like asking every backend for it's user count) in migrations.
2: The current check is wrong because countUsers returns an array. var_dump([] > 1000) => true ;)

Signed-off-by: Daniel Kesselberg <mail@danielkesselberg.de>