Thanks to inferenceus : some sort orders on the "explore/users" page
could list users by their lastlogintime/updatetime.
It leaks user's activity unintentionally. This PR makes that page only
use "supported" sort orders.
Removing the "sort orders" could also be a good solution, while IMO at
the moment keeping the "create time" and "name" orders is also fine, in
case some users would like to find a target user in the search result,
the "sort order" might help.
wxiaoguang