We're going to upgrade the Sentry gem in a future MR. The new version
exports a Sentry class, rather than the current Raven class. This would
clash with our existing Sentry namespace, used for the Sentry::Client
class, which is part of our feature where user projects can integrate
with Sentry.

To start to untangle this, we move Sentry::Client to
ErrorTracking::SentryClient. This has a number of benefits:

1. We're following our namespacing guidelines by putting this code under
   the ErrorTracking namespace, which is the relevant application domain.
2. We don't end up with any Sentry namespace at all, so there is no
   ambiguity when we add the top-level Sentry definition with the gem
   upgrade.
3. The indentation for most classes and modules in this commit doesn't
   change, leading to a cleaner diff :-)

There are also some classes related to the error tracking feature that
are in the Gitlab::ErrorTracking namespace, rather than just
ErrorTracking. Those are tracked in the below issue:

#323342