Allow for the default number of search results to be configurable

Overview

At the present moment the default number of results to be returned on a search is hard coded to 50 in CRM/Utils/Pager.php this should ideally be configurable using settings

Example use-case

A site wants to show 100 contacts / memberships / grants etc on a screen at a time rather than 50

Current behaviour

Hard coded default value of 50

Proposed behaviour

Create a setting to handle the default value and set it on various search forms.