Add the VisitType to the VisitContextModel class

The VisitContextModel is a very simple view of a visit, suitable for use in an app contextModel.
For now, it doesn't include the Visit Type of the specific visit.

Add the VisitType to the model in a way that is consistent to the JSON REST representation of objects, so it can be used in pages.
See https://talk.openmrs.org/t/how-to-conditionally-display-forms/4197/4

Resources:
https://github.com/openmrs/openmrs-module-coreapps/blob/master/api/src/main/java/org/openmrs/module/coreapps/contextmodel/VisitContextModel.java