Support ability to limit scopes by application

If there no scopes (empty string) defined in the Doorkeeper::Application record, PreAuthorization should only compare scopes against the config/server scopes. If there are scopes on the Doorkeeper::Application record, the set of valid scopes is the intersection of the config/server and the Doorkeeper::Application record scopes.
Merge pull request #469 from cpapazian/feature/application_scopes

Client application specific scopes