When a blocked user tries to log in they are immediately signed out
again.

This in turn calls the `before_logout` callback in Warden. In that
callback we try to log the activity for the blocked user in the
`BlockedUserTracker`:
https://gitlab.com/gitlab-org/gitlab/blob/e30d909d3f19511742d218da0b4bd2cecca8b7a2/config/initializers/warden.rb#L60

If a `SystemHook` was configured, this would also mean scheduling a
job. Scheduling a job tries to get the current user from the
context. For the `OmniAuthCallbacksController` this would call
`current_user` which would try to sign the user in again.

The `OmniauthCallbacksController` is an `ApplicationController`, which has
a `#context_user` method that uses `#auth_user` which also exposes
`#current_user`, but it has a safeguard to not call current user
multiple times.

Instead of trying to fetch it again for the omniauth sign in
request. We push it into the context when the sign in was valid.

Changelog: fixed