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Use ujson instead of json in the api package

- Add ujson to requirements
- import ujson as json in all the sa_api and sa_manager modules
- Update dumps calls to not use the indent argument
- Update the tests to not expect wrapped, indented JSON
Fix: API key optimization

- Use select related on the model manager instead of the instance returned from the get method (model instances don't have a select_related method; it must be called before the query is run)
Merge branch 'master' into dj15