refactor importer config to be createable without a request object
refactor the case import

main changes are:

- separating out the celery / download from the part that does work so
  that it can be readily callable from other parts of the code
- submit caseblocks in bulk rather than one at a time (optimization)
- moving stuff around to make that workflow a little more manageable
unused import
fix bug where named columns was never actually set to false
initial tests for importer
more tests and tweaks to support them
make chunksize configurable and return as part of the metadata back

+ basic test to validate chunking (and another test that snuck in)
optimizations and tests for external_id/chunking code
Merge pull request #1929 from dimagi/importer-speedup

Importer speedup