Move Mongo submission access code to kobocat_backend

[jnm]

Move this:

kpi/kobo/apps/reports/report_data.py

Lines 19 to 27 in 7a30668

	def get_instances_for_userform_id(userform_id, submission=None):
	query = {'_userform_id': userform_id, '_deleted_at': {'$exists': False}}
	if submission:
	query['_id'] = submission
	instances = settings.MONGO_DB.instances.find(query)
	return (
	MongoDecodingHelper.to_readable_dict(instance)
	for instance in instances
	)

...to https://github.com/kobotoolbox/kpi/blob/7a306680a7c7f34490d5eb95e6d0c1646fc02a3b/kpi/deployment_backends/kobocat_backend.py

MockDeploymentBackend has a _get_submissions() method; perhaps KobocatDeploymentBackend should have one as well (but why is it marked private?):

kpi/kpi/deployment_backends/mock_backend.py

Lines 59 to 60 in 7a30668

	def _get_submissions(self):
	return self.asset._deployment_data.get('submissions', [])

Requirements for submission data access (could be spread across multiple methods):

• Ability to get individual submissions
• read json from mongo and decode it (or, certainly decode it before sending to an external REST server, if there’s some reason not to always decode it as soon as it’s read from Mongo)
• read xml from postgres (readonlyinstance)