Ability to skip keys if their value is None for content exporter

Ability to skip keys if their value is None for content exporter
Increased page size of SAPSF inactive user results from 500 to 1000

Fixed Server Error on enterprise course enroll url

Fixed Server Error on enterprise course enroll url caused by week_to_complete None value

Fix error calling /search/all/ endpoint

Merge pull request #505 from edx/bessiesteinberg/bump-to-1.6.15

Bump Version to 1.6.15

1.6.13: Merge pull request #501 from edx/ziafazal/ENT-2009

ENT-2009: Improved logging of unlink_inactive_sap_learners command and match social auth user by `uid` field

1.6.12

Update ap clients to call the correct site (#502)

1.6.11

Update the format of course_duration in xAPI payload data

1.6.10

• Remove old catalog model field.

1.6.9

Merge pull request #498 from edx/tuchfarber/ent-2015

Change DjangoFilterBackend import location

Fix error in enrollment flow caused by course key parsing

Merge pull request #497 from edx/bessiesteinberg/bump-version-number

Bump version to 1.6.8

added enable_audit_data_reporting in EnterpriseCustomerSerializer

added enable_audit_data_reporting in EnterpriseCustomerSerializer