[Merge] lp:~phablet-team/camera-app/watch-refresh-thumbs into lp:camera-app

[Florian Boucault]

> "?at=" + Date.now() makes it so that the thumbnail is _always_ recomputed even
> though the file might not have changed.

Not sure about that anymore, my brain is failing. Let's discuss it ASAP
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https://code.launchpad.net/~phablet-team/camera-app/watch-refresh-thumbs/+merge/247837
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