Rework Export Batch Functionality

[aembler]

Right now, the core exportPage() method renders the page, in order to make sure that all the areas get included properly in the export. Unfortunately, attempting to render the dashboard after doing this causes problems. We didn't notice this before because we never ran the View::render() method. We need to refactor export so that it does the following:

1. Upon visiting the export page, we check to see if export XML already exists. If it does, we just list the files and provide a link to the XML.
2. If the XML file doesn't exist, we generate the XML, save it to the files directory, and refresh the page. This will stop at Incorrect theme path? #1 above.
3. Add a "Refresh" button to the page, which will remove the XML, and refresh the page (which will then trigger step no. 2 above.)

[aembler]

This should fix #1 and #2