Sentry frontend/backend tracing support

[liamwhite]

If you deploy Sentry in production, I would suggest updating your privacy policy to add the following:

• Client-side scripting errors are collected when they previously were not; you should state that error reports are sent from the browser when this happens. (TODO: add client opt-out)
• If your DSN is sentry.io, then you should add your DPA with Sentry / Functional Software, Inc.
• If your DSN is self-hosted, then you don't need to add anything about a DPA