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
IHMO we should:
- Merge `Version` and `File` models
- Move anything that can be changed in a specific package to `Version`
- Leave `AppFeature` separate, they are already tied to a `Version`, and fairly specific, that's fine.

But I'm completely off-topic. For this particular field, I think it's fine on `Addon`.