Use Pelican to build the index file for the app theme

Right now the app theme is a handcrafted index.html file that uses JavaScript to load each one of the individual pages and insert the HTML into it (*).

While that worked very well for the first release, it'd be good to have a more modular approach to use Pelican templates to build the HTML and drop that bit of the JavaScript code. It seems we could do that with a custom template and using the PAGES variable to iterate through each page and insert their {{ page.content }}.

This depends on bug 1434405 being fixed.

(*) Note: the requirement to have all content in one page for the app theme still remains, as the tabbed interface for the phone needs to have all content in one place.