Snap Backend: List available updates and allow to trigger them manually
This MR addresses three issues in the context of Snap upgrades:
- the returned size information would be wrong or missing in some cases
- the available updates wouldn't be listed, you'd have to go to an app page look at the version and compare to the one reported by the current channel to figure out it wasn't up to date
- the updates couldn't be triggered to be applied
Now it is possible to list available update and apply them.