use apps iframe for installs/launch (bug 1029687)

Use the iframe to:

- install apps
- query for installed apps
- launch apps

In market_button.html, check if the app is installed to render the right text rather than asynchronously updating the button after the app is rendered.
not sure we need inheritance. just add the helpers to the nunjucks global context. just like how we do with `helpers.py` files in zamboni.