Use Gitlab.config instead of Settings everywhere

They are the same because of https://github.com/gitlabhq/gitlabhq/blob/2b816075dc71dfe8f6f9e5349fdff7f03ad9dad0/config/initializers/2_app.rb#L5, but `Gitlab.config` is used far more than `Settings`, so let's use it everywhere.

I wonder why though: `Settings` is shorter, easier to understand since it is the same for every app that uses settings logic. Took me a while to find that 2_app.rb file.