Setup onUpdateAvailable handler #73
Description
We should listen for the chrome.runtime.onUpdateAvailable event.
This will let us do some cleanup for extension updates, like telling any open quick reply panes to submit as a preview so messages-in-progress aren't lost on extension update. There's undoubtedly other cleanup we can attempt with content scripts as well.
It also could be used to clean up inserted CSS from #72 - at least on Firefox until https://bugs.chromium.org/p/chromium/issues/detail?id=608854 is resolved on Chrome.