[TRACKING ISSUE -API] Add New Hangouts Endpoint

[BethanyG]

Add a Hangouts api endpoint.

This is a Tracking Issue, individual task issues linked:

working branch: issue-160-hangouts-endpoint

[ ] Create JSON spec for hangouts api endpoint: #161
[ ] Create sample data/mocked data fixture for hangouts api endpoint: #162
[ ] Create hangouts app, apps.py: #163
[ ] Create hangouts model & associated DB migration files: #164
[ ] Create hangouts views & queries: #165
[ ] Create hangouts serializer #166
[ ] Create hangouts admin view and register app with Django admin interface #167
[ ] Create hangouts URLs (api/v1/hangouts, and any others needed) #168
[ ] Ensure associated unittests and test data are created for all code changes
[ ] Document expected GET, POST, PATCH behavior and any other endpoint-specific docs

Acceptance Criteria:

All items checked off above, and all associated PRs reviewed, accepted, and merged.

[lpatmo]

Thank you for creating these issues! 👏