Integrate Sentry.io error reporting

[francisli]

Post in #intentional-walk-dev-chatter with an email address to be invited to our project in Sentry.io.

Then, integrate the Python/Django error reporting code as per their getting started guide.

https://sfcivictech.sentry.io/projects/intentional-walk-server/getting-started/?product=performance-monitoring&product=profiling

[JimmiHagen]

We will also want to instrument the app itself - this gives insight into on-device errors and may be more useful in this project than server-side logs : https://docs.sentry.io/platforms/react-native/#install
but that's a separate issue

[JimmiHagen]

changes are pushed to branch
tests run, only issue is known, related to that other branch I should have fixed up by now
=========================================================== short test summary info ============================================================
FAILED home/tests/unit/api/test_appuser.py::TestIsTester::test_is_tester - AssertionError: True != False : failed 'iwt-d'
================================================== 1 failed, 79 passed, 17 warnings in 23.97s ==================================================