Add incompatibility banner when user agent doesn't support navigator.mozApps (bug 878329)

r? @cvan @mattbasta

## Screenshots

### At 320px
![320](https://f.cloud.github.com/assets/23885/1424816/bd7e276a-4026-11e3-803a-a383f584d02d.png)

### At 709px
![709](https://f.cloud.github.com/assets/23885/1424818/bfe8c460-4026-11e3-9e8b-9a8f8c7f4447.png)

### At 1023px
![1023](https://f.cloud.github.com/assets/23885/1424819/c1c208fa-4026-11e3-9069-6e1dee96a99a.png)

### At 1280px
![1280](https://f.cloud.github.com/assets/23885/1424820/c40bb85e-4026-11e3-83f6-e16696bc09b0.png)