mv mappings to json file; mod func to handle doc and facet mappings

- Moved the mappings data structure out to a JSON file. This file
  contains a complete mapping for the current facets, and stubs
  for the document types. Document field properties are pulled in
  from the model fields documents.
- Modified the _make_mappings() func to accomodate facet mappings.
expanded topic, facility; data for each facet in separate file

- Added more detailed topic and facility facet data.
- Broke up the facet.json file into separate file for each facet
  (topic and facility are big files).
- Facet terms are now dicts instead of tuples.