[Merge] lp:~brian-murray/ubuntu-release-upgrader/deb-to-snap into lp:ubuntu-release-upgrader

[Steve Langasek]

The changes all look good; I think we still need to sort out the timeout on the snap downloads before pushing this through.
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https://code.launchpad.net/~brian-murray/ubuntu-release-upgrader/deb-to-snap/+merge/348916
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