Move case importer work to celery

Make use of Soil to offload the importing of cases to a backround task
and be able to provide the user with a progress bar while the work is
happening.
Prime views on long running import jobs

Remove the old 500 line excel file restriction and instead replace it
with a hit to prime the views every 500 cases.
Spruce up priming view logic in import

Prevent back to back priming (giant slowdown) and use actual changes to
data to decide when we've processed enough entries to fire the cleanup
Add trailing slash to url
Move prime view method to dimagi-utils
Merge pull request #382 from dimagi/celery

Move case importer work to celery