Sort submissions and places by reverse updated time by default
Join with dataset owner when getting places and submissions, to avoid extra queries
Make query errors more specific
Add CORS support
- Add Access-Control-Allow-Origin: * response header to all API requests
- Add a client authenticator that uses the request Origin header for permissions
- Add the OriginPermission model for associating origins with datasets
Unset default ordering when grouping on dataset and submission set name
Use simpler filter instead of extra query method
Fix submission grouping

Using and not overriding a default sort ordering breaks some annotations. The ordering parameter may not be included in the grouping, though Django will always try to add it
Fix the tests vis a vis the default ordering
Clean up tests
- Get rid of deprecation warnings by giving all datetimes tzinfo
- Comment out unused print statements
Allow requesting of multiple specific places and submissions at once
- Add an optional comma-separated list of primary keys to the place and submission routes
- Use the values in the pk list to filter the places/submissions queryset