Move Sentry processors to before_send hook [RUN ALL RSPEC] [RUN AS-IF-FOSS]

We want to upgrade the version of the Sentry gem we're currently using to the latest version. However, this makes some breaking changes. One of the biggest for our purposes is that it no longer supports processors - these were classes to pre-process the event hash before sending to Sentry.

In the latest version, these are no longer available, but we can still use any logic we like in a custom before_send hook. So we can manually chain our existing processors there. However, there's a catch: processors in the current version take an event hash, but the before_send hook takes an Event object. (Raven::Event in the current version, Sentry::Event in the new one.)

This change adds a feature flag - sentry_processors_before_send - that defaults to off. When it's off, we use the processors as we do now. When it's on:

1. All the processors become no-ops when called as processors.
2. We enable them in a before_send chain through a different interface.

Because they take a different type of argument, this makes the processors in this change a little ugly. We can tidy that up once we've validated this feature flag in production, even before upgrading the gem. We also add specs to test each processor both with and without the feature flag enabled.

For gitlab-com/gl-infra/scalability#849 (closed).