Simplify environment role updates in app settings.

Use ApplicationRole.id instead of User.id in forms. This eliminates the
need for the function that checks whether a user is in a given
application, because looking up the application role will raise an error
if the user is not.

atst/forms/app_settings.py

@@ -6,7 +6,7 @@ from .data import ENV_ROLES, ENV_ROLE_NO_ACCESS as NO_ACCESS
 
 
 class MemberForm(FlaskForm):
-    user_id = HiddenField()
+    application_role_id = HiddenField()
     user_name = StringField()
     role_name = RadioField(choices=ENV_ROLES, default=NO_ACCESS)