fix: resolve customAccessToken warning error that causes app to crash

In the present implementation verifyUserRelations(Model) sets a const hasManyTokens which seems to resolve to a boolean or undefined, so checking hasManyTokens.polymorphic throws an error.

It seems the intention is to check whether the the following conditions are met:
 1. a model has a hasMany accessTokens relation
 2. that relation is polymorphic

The proposed change should resolve correctly for any models that meet this criteria.

lib/application.js

@@ -460,7 +460,7 @@ app._verifyAuthModelRelations = function() {
     const hasManyTokens = Model.relations && Model.relations.accessTokens;
 
     // display a temp warning message for users using multiple users config
-    if (hasManyTokens.polymorphic) {
+    if (hasManyTokens && Model.settings.relations.accessTokens.polymorphic) {
       console.warn(
         'The app configuration follows the multiple user models setup ' +
           'as described in http://ibm.biz/setup-loopback-auth',