Add Sentry as supported incident system.

[rikkert]

This adds support for Sentry error tracking as incident input. Not sure if this needs more documentation in the readme. The bug clause for sentry in incidents.sql probably needs tweaking / use case but this seems like an ok starting point. The metadata doesn't include any kind of commit sha or release information atm. Not sure how to make this count in the failed deployments graph without this info.