We also try to unify the way we setup OmniAuth, and how we check
if it's enabled or not.

it also means we're using omniauth, so we need to set it up.

Which could extend from EE

* Replace `require` or `require_relative` with `require_dependency`
* Remove unneeded `autoload`

Adds a rubocop rule (with autocorrect) to ensure line break after guard clauses.

This avoids loading the `OmniAuthCallbacksController` at boot time so
it doesn't mess up the `before_action`-chain