feature(settings): adds setting for default number of items per page

Most queries that default to 10 now use the setting, whose valid range
is, for now, 2 to 5000. In order to keep this simple, queries that
default to 20 items will continue showing at least 20 items; the setting
is used only if it’s greater than 20.

Fixes #2650

* [x] Add upgrade script
you can drop this limit as elgg_list_entities will already fallback to default behaviour