trace: Migrate to sentry-go

Created by: keegancsmith

raven-go has been deprecated for a while. This migrates us to the blessed package for sending events to Sentry. It works in a strange way for Go were it seems to simulate goroutine local storage. Notice that the ConfigureScope and CaptureException functions both do not rely on any sort of context. There doesn't seem to be any alternatives which work with less magic :(

Additionally sentry-go always does some work, even if a request doesn't report an exception. It will always clone a hub for a request, and will always create a sentry.Request (with a copy of headers).

This commit also I removes the reportError function from handlerutil. It didn't seem to get used in practice (our production sentry has no hits for the tags it sets).

Additionally we move some "tags" to "extras". Tags are better suited for low cardinality values (the values have stats calculated on them).

Finally I improve the initialization such that we only configure sentry if sentry related configuration changes.

Part of https://github.com/sourcegraph/sourcegraph/issues/6738