- Previously, AccessTokenValidationService was a module, and all its  public
methods accepted a token. It makes sense to convert it to a class which accepts
a token during initialization.

- Also rename the `sufficient_scope?` method to `include_any_scope?`

- Based on feedback from @rymai

- Move the `Oauth2::AccessTokenValidationService` class to
  `AccessTokenValidationService`, since it is now being used for
  personal access token validation as well.

- Each API endpoint declares the scopes it accepts (if any). Currently,
  the top level API module declares the `api` scope, and the `Users` API
  module declares the `read_user` scope (for GET requests).

- Move the `find_user_by_private_token` from the API `Helpers` module to
  the `APIGuard` module, to avoid littering `Helpers` with more
  auth-related methods to support `find_user_by_private_token`