Setup Sentry crash reporting for frontend code

We use [Sentry](https://sentry.io/) for tracking errors in both backend (Python) and frontend (JS) code.

I have created a Sentry project for the lms app's frontend code. The "public DSN" configuration parameter should come from a `SENTRY_DSN_FRONTEND` env var. Search for "SENTRY_DSN_FRONTEND" in the h app to see how we do it there.

The setup steps are here: https://docs.sentry.io/clients/javascript/install/ . Just the basic error capture should be fine, no framework integrations are needed.