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 predicatble 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.
Ensure PageIds are part of elements when created by JS page classes, otherwise the display cannot be saved.
When calculating the intersection of displays prioritize the last display added.  This means that element weights from the last display will get priority.
Fix bad merge on DisplayManager.class.js

For some reason the rebase on this file didn't complete and merge markers were left in.
Add the displays in the correct order
AdminController:listDisplays was not correctly adding displays to the intersection to the weights were being lost.
Merge pull request #248 from jrjohnson/downloadbydisplay2

Download Excel using display