NEW Enforce max node counts to avoid excessive resource usage

Rendering potentially 1000s of nodes can exceed the CPU and memory constraints
of a normal PHP process, as well as the rendering capabilities of browsers.
Set a hard maximum for the renderable nodes, deferring to a "show as list" action
in the main CMS tree. For TreeDropdownField, we don't have the list fallback option,
so ask the user to search for the node title instead.

Also makes both the "node_threshold_total" and "node_threshold_leaf" values configurable