[feature] Add created_at and error_description to /oauth/token endpoint (#645)

* start fiddling about with oauth server

* start returning more helpful errors from oauth

* test helpful(ish) token errors

* add missing license header

internal/api/client/auth/authorize.go

@@ -246,7 +246,7 @@ func (m *Module) AuthorizePOSTHandler(c *gin.Context) {
 		sessionUserID:       {userID},
 	}
 
-	if err := m.server.HandleAuthorizeRequest(c.Writer, c.Request); err != nil {
+	if err := m.processor.OAuthHandleAuthorizeRequest(c.Writer, c.Request); err != nil {
 		api.ErrorHandler(c, gtserror.NewErrorBadRequest(err, err.Error(), helpfulAdvice), m.processor.InstanceGet)
 	}
 }