Part of https://gitlab.com/gitlab-org/gitlab-ce/issues/61578

Rails is slow to generate paths dynamically especially when called
hundreds/thousands of times. Also, rendering many partials hundreds
of times can be slower. This change reduces the number of partials
rendered and introduces two fast path methods to speed up path
generation.

The initializers including this were doing so at the top level, so every object
loaded after them had a `current_application_settings` method. However, if
someone had rack-attack enabled (which was loaded before these initializers), it
would try to load the API, and fail, because `Gitlab::CurrentSettings` didn't
have that method.

To fix this:

1. Don't include `Gitlab::CurrentSettings` at the top level. We do not need
   `Object.new.current_application_settings` to work.
2. Make `Gitlab::CurrentSettings` explicitly `extend self`, as we already use it
   like that in several places.
3. Change the initializers to use that new form.