Default user listing pagination fails sometimes

Sometimes, when loading the Users page, the results per page will be set to "10", while for example all 22 users are shown. Once you select a different number of results per page, the two will be aligned again.

My theory is that the default number of results per page, which is 50 since !163 (merged), is actually not available if the number of users is less than that, and somehow if the default is invalid the select box will just pick the top entry while the value is unset, or maybe 50 even though that's not a provided option. So we should adapt the default to the number of users.