Push appcache updates when available

[ewinslow]

Since the app never actually refreshes the page, we added appcache polling to pull down updates. But once the update is ready, the user still needs to refresh the page in order to get the changes.

Some suggestions for this are:

1. show a modal asking the user to download the new version
2. show an update indicator which the user can click to refresh the page
3. force refresh when the user navigates URLs

1 has got to be the easiest to implement but also worst user experience. We don't want to interrupt people in the middle of whatever they're doing.

2 is subtle and nice, but is still trying to draw their attention away from what they're currently doing.

3 is my favorite because it catches them when they're already in a transitionary mindset. They're not in the middle of reading or typing, but rather expecting the page to change, just like a traditional full-page-refresh webapp.

If there are any other ideas for when we could do this, I'm open to hearing them, but this seems like the best opportunity.