Allow configuring of max graphs per page

It would be convenient to allow the configuration of the ZBX_MAX_GRAPGS_PER_PAGE setting in the UI.  I feel this would be most helpful as a system-wide default, though I can see how others might want it as a user-specific override as well.

This is of particular importance to us because our Zabbix system is a little under powered, and wasting the resources to generate upwards of 19 unneeded graphs every time someone opens the page is painful to our system.