Replace .push with .concat to port to Mongodb 4

Otherwise actions such as commenting fail with:

Unknown modifier: $pushAll

As per: https://stackoverflow.com/questions/48607918/mongoerror-unknown-modifier-pushall-in-node-js/50435618#50435618

Fix gothinkster#57
Fix gothinkster#96
Fix gothinkster#64

models/User.js

@@ -60,7 +60,7 @@ UserSchema.methods.toProfileJSONFor = function(user){
 
 UserSchema.methods.favorite = function(id){
   if(this.favorites.indexOf(id) === -1){
-    this.favorites.push(id);
+    this.favorites = this.favorites.concat([id]);
   }
 
   return this.save();
@@ -79,7 +79,7 @@ UserSchema.methods.isFavorite = function(id){
 
 UserSchema.methods.follow = function(id){
   if(this.following.indexOf(id) === -1){
-    this.following.push(id);
+    this.following = this.following.concat([id]);
   }
 
   return this.save();

routes/api/articles.js

@@ -254,7 +254,7 @@ router.post('/:article/comments', auth.required, function(req, res, next) {
     comment.author = user;
 
     return comment.save().then(function(){
-      req.article.comments.push(comment);
+      req.article.comments = req.article.comments.concat([comment]);
 
       return req.article.save().then(function(article) {
         res.json({comment: comment.toJSONFor(user)});