Download Excel using display

On both the grid and applicants download allow the user to select a display and download applicants using it.
Use an interface for Display\Element class and get/set for all of the properties. This allows us to list elements in a predictable way for output.
DisplayElements of type 'element' also return a pageId.
Make Entity\Application::getApplicationPageByChildPageId recursive so it can get any child page.
Remove CSVPage interface and use displays and answer array to allow any page type to create a CSV value. The csv will be ordered just like the original display.