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.
well you could use https://marketplace.allizom.org/iframe-install.html/ on Stage and https://marketplace-dev.allizom.org/iframe-install.html/ on Dev. that doesn't sound too hard. but I'd let @krupa, et. al. know.