Speed up the api and api usage

This PR speeds up the apis by removing the dehydrate cycle for the get list of objects.
The Tastypie speed decreases and the increase of the number of data due to the heavy processing is in that step, like adding external relations and altering fields. 
The advantage of this pr is that the apis are very fast now on the other hand we loose some information that was created on the post process. For this purpose a couple of new fields have been added to resourcebase to store the thumbnail_url and the absolute_url so that we can retrieve them directly from the db without any elaboration. This is a denormalization of the db, unfortunately.

There are also a couple of improvements in the category, keyword and counts.