FIX Respect tree node limits, fix search result node display

- Renamed $minNodeCount to more accurate $nodeCountThreshold
- The $minNodeCount attribute wasn't properly respected
during actual querying, so SilverStripe would always traverse
the entire tree (and load all objects into memory),
before then marking nodes as "unexpanded", which prevents
them from actually being rendered.
- Fixes nodes on search results to be expanded by default
- Fixes nodes on search results to correctly ajax-expand
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