Add schema type option for module search constraints

[ahayes]

Nunaliit's search UI is currently installed by default and searches all documents.

Currently, an Atlas Builder can disable and/or constrain this search behaviour by adding a search object whose contents are described in the Module Document documentation thusly:

If specified, this object is used to provide configuration information to the search service. The attribute "disabled" (boolean) can be defined, which specifies if the search bar will be included in the display of the module. The attribute "constraint", if specified, can be a layer string (i.e. a single layer constraint), an array of layer constraints, or an object with a type of either "layer" or "model" with an array of strings which identify the constraint imposed on the search results (either "layerNames" or "sourceModelIds"). E.g. If layer identifiers are specified, then only documents posted on those layers are reported in search results.

Being able to also constrain by one or more schemas would be helpful. For backwards compatibility, this should be implemented as a new type choice of schema for the constraint object with a schemaNames array.