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.
- Use `GlobalPolicy` to authorize the users that a non-authenticated user can
fetch from `/api/v4/users`. We allow access if the `Gitlab::VisibilityLevel::PUBLIC`
visibility level is not restricted.
- Further, as before, `/api/v4/users` is only accessible to unauthenticated users if
the `username` parameter is passed.
- Turn off `authenticate!` for the `/api/v4/users` endpoint by matching on the actual
route + method, rather than the description.
- Change the type of `current_user` check in `UsersFinder` to be more
compatible with EE.
