add %AdminSearchResultRows config option

This adds a row limit to the Classes and Templates list for the first time.

It also changes the row limit for Groups and Users lists from 100 to 50.

The rows limit for the Membership page for Groups and Users is changed from
20 to 50. This e.g. helps adding groups to users as it avoids jumping through
pages if you have many groups.
Merge branch '4.2/new-admin-search-result-rows-config' into 4.2-trunk