API support for purgeConceptNameTag method

The purgeConceptNameTag method does not exist in the core api. It needs to be added.

The following functionality is being added to the openmrs-module-conceptmanagement for now:
If the user wants to get rid of a ConceptNameTag we just delete it (and delete all rows from concept_name_tag_map that reference it).
purgeConceptNameTag needs to delete the tag from the DB as well as all existing usages of the tag.