Migrate from raven to sentry-sdk

[Aramgutang]

Raven officially only supports Django 2.x, and will not be receiving updates, so it's liable to lose compatibility at some point. We should preemptively migrate to sentry-sdk to avoid issues, which is still compatible with the old API used by our self-hosted Sentry.

Docs for migration can be found here: https://docs.sentry.io/platforms/python/migration/

[aweakley]

I ran into this just now on a project where I'm upgrading Django- there were no complaints from raven that I could see, but no error reports coming through to Sentry until I switched to sentry-sdk.