App should warn if downloading large files over mobile network

[intrications]

The app should check if it is downloading over the mobile network because users might be charged per MB downloaded.

There should be an option to download over WiFi only and the app should wait for WiFi before downloading the file.

This especially applies to issues automatically downloaded from a push notification.

[libreliodev]

Yes, there should. By default, issues should not be automatically downloaded when no wifi is available.

User settings could be defined using a form as specified in #221.

[libreliodev]

@intrications

When the user downloads a paid issue, the app is given a temporary URL to download from Amazon S3. How long is that URL valid for? If the app may wait for WiFi until downloading then we may need to detect an expired URL and refresh it.

The URL is only valid for a few minutes. So we should refresh the URL in any case if the app has been waiting for the Wifi

[intrications]

This relies on #232