Offline checkbox 1049770

https://bugzilla.mozilla.org/show_bug.cgi?id=1049770

The management command doesn't reindex the apps, I figured we could just index everything after we run it. It only sets apps with an appcache to `is_offline` per @diox's suggestion.

![screenshot 2014-11-20 14 44 59](https://cloud.githubusercontent.com/assets/211578/5132622/ed8c7e9c-70c3-11e4-9cc8-5624ceefc32e.png)
> App submission

![screenshot 2014-11-20 14 45 32](https://cloud.githubusercontent.com/assets/211578/5132623/ed9bc212-70c3-11e4-80bb-0ad0363d715e.png)
> View technical

![screenshot 2014-11-20 14 45 42](https://cloud.githubusercontent.com/assets/211578/5132624/edd09cd0-70c3-11e4-8608-96216540becf.png)
> Edit technical
Yeah, this is the kind of stuff that's usually done with `process_addons` so that it can be chunked and processed by the celery workers.

Note: according to https://marketplace.firefox.com/search?q=%3Ahosted+%3Aoffline we have 398 matching public apps that'll get this flag. Not bad!