Performance improvements

Before, the configuration file elements.yml was read multiple times when, for example, editing a page. We ran in to some performance issues. Loading the edit page view took > 10 seconds. After this fix, loading the edit page view now takes less than 1 second. 

This commit will read elements.yml once and save it in memory. A module was created that handles the reading of the file, just as in the case for page_layout.yml. 

The obvious drawback with the solution is that a developer has to restart the server every time elements.yml is changed. Apart from that, it'll speed up the application quite a lot.