feat: initialize agreements app

Adds an app to openedx for the Agreements feature.
Includes an IntegritySignature model with a basic
API, as well as a waffle flag to support rollout.

openedx/tests/settings.py

@@ -82,6 +82,7 @@ INSTALLED_APPS = (
     'openedx.core.djangoapps.theming.apps.ThemingConfig',
     'openedx.core.djangoapps.external_user_ids',
     'openedx.core.djangoapps.demographics',
+    'openedx.core.djangoapps.agreements',
 
     'lms.djangoapps.experiments',
     'openedx.features.content_type_gating',