MITRE refactor pull request

* Renames AuthorizationRequest to OAuth2Request, and changes interface to be a single concrete class with accessible getters and setters
  * Removes DefaultAuthorizationRequest
* Renames AuthorizationRequestManager to OAuth2RequestFactory, and moves validation method to OAuth2Utils
* Updates the UserApprovalHandler interface to have two distinct methods: checkForPreApproval and updateAfterApproval, to cleanly distinguish between the two times it is called
* Simplifies the flow of the AuthorizationEndpoint