Merge branch 'collection_thumbnails' into 'master'

Changed collection item thumbnails to show only not blank thumbnails

See merge request !363

app/serializers/collection_serializer.rb

@@ -15,8 +15,11 @@ class CollectionSerializer < ActiveModel::Serializer
 
   def items_thumbnails
     thumbs = []
-    object.collection_items[0..3].each do | t |
-      thumbs << t.thumbnail
+    i = 0
+    while thumbs.size < 4 && i < object.collection_items.size do
+      t = object.collection_items[i].thumbnail
+      thumbs << t unless t.blank?
+      i=i+1
     end
     thumbs
   end