Indicate whether applications support offline (bug 861303)

https://bugzilla.mozilla.org/show_bug.cgi?id=861303

Screenshots:
![Mobile, unavailable app](https://cloud.githubusercontent.com/assets/187006/3148198/dfc2f2f6-ea5d-11e3-8a4c-1dc83dcdfbe9.png)

![Mobile, regular app](https://cloud.githubusercontent.com/assets/187006/3148197/dfc014be-ea5d-11e3-94a0-670c00191468.png)

![Desktop, regular app](https://cloud.githubusercontent.com/assets/187006/3148199/dfc567d4-ea5d-11e3-815e-26e8e44fe2ad.png)

More screenshots to show that the changes to the `.install` button style should be safe:
![Desktop listing](https://cloud.githubusercontent.com/assets/187006/3148200/dfc6dd8a-ea5d-11e3-8d07-483f4b2d74b0.png)

![Mobile listing](https://cloud.githubusercontent.com/assets/187006/3148201/dfc9a1fa-ea5d-11e3-9f06-f3ca10e37a3e.png)

why not just add a `class="hidden"` and then toggle it?