Changed database export's use of views (7155)

- Generally refactored much of export_database.py; made it use
  couch module for views.
- Refactored couch.Couch methods that query a provider's docs,
  and use the built-in _all_docs view.
- Added couch.Couch.dpla_view and couch.Couch.all_dpla_docs
- Removed dpla database view all_provider_docs/by_provider_name
- Removed dpla database view export_database/all_source_docs
- Added dpla database view export_database/profile_and_source_names