Trial running Sentry in Admin

This will capture and send various events to Sentry:

- Any unhandled exceptions.
- Any logger.error calls.
- Some request traces.

The latter are severely limited to avoid going over the free tier
limits for Sentry, and to avoid excess effort on our end.

requirements.in

@@ -41,3 +41,5 @@ cryptography<3.4 # pyup: ignore
 # version 0.10.0 introduced exceptions when workers crashed due to deprecating lower case `prometheus_multiproc_dir`.
 prometheus-client>=0.9.0,!=0.10.0
 gds-metrics==0.2.4
+
+sentry-sdk[flask]