Allow viewing alert detail offline

[justdave]

When in low/intermittent bandwidth situations, it's possible to download the index of current alerts and then be without Internet at the point when you are reading them. The app should offer the option to proactively download and save the alert text as the alerts are downloaded, so the cached copy can be viewed. This would also be a UI performance improvement, as the alert text would display immediately instead of having to watch for it to be fetched when you tap on the alert.