heavy refactor of vocab import/export

You can now move data between JSON and CSV formats with no loss.
Tested with Google Docs.
Works okay with smaller taxonomies but will need a refactor for
a larger vocab.
don't do models-specific processing on facets mappings
get mappings and facet data from Repository repo by default

The mappings, facet data (including topics terms) and model fields
definitions should not be part of the source code.
Rather they should be in repositories closer to the data.
Merge branch 'vocab-unified' into develop