Use api user id instead of the provider's id for a user when saving oauth tokens

src/main/scala/code/snippet/OAuthAuthorisation.scala

@@ -88,7 +88,10 @@ object OAuthAuthorisation {
                   //link the token with the concrete API User
                   obpUser.user.obj.map{
                     u  => {
-                      appToken.userId(u.id_)
+                      //Note: ApiUser.id is the database id from ApiUser, as opposed to ApiUser.id_, which returns the id assigned
+                      //to this user by its provider (i.e. not this api). We want ApiUser.id because it is unique, unlike the id given by a provider
+                      // i.e. two different providers can have a user with id "bob"
+                      appToken.userId(u.id.get.toString)
                     }
                   }
                   if (appToken.save())