Ordering

Various models support ordering, such as Provisioning Targets. Standardize and document a way to easily enable this on a per-model basis:

1. schema.json defines the ordr field (with value inherited from the schema library)
2. The model adds OrderableBehavior, which automatically injects the new maximum value for the row
3. columns.inc enables drag-and-drop reordering, either via a flag or perhaps automatically if the column ordr is defined in $indexColumns
4. StandardController implements any necessary callback(s) for drag-and-drop (similar to how provision is implemented there for any provisionable models)

(It's not clear if this should apply to primary objects that also support order values, such as PersonRoles.)