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

(replaces #7339 -- trying to squeeze this in)

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
Because of engine/lib/upgrades/2014130300-1.10.0-add_default_limit-fcef9e7ce01e26a4.php