[Merge] lp:~sil2100/ubuntu-release-upgrader/snap-size-estimation into lp:ubuntu-release-upgrader

[Łukasz Zemczak]

Was also looking into making DistUpgradeCache mentioning the not enough space being because of extra_snap_space, but from what I see the way it's written right now feels very generic and not sure if I should change that. But I'll look into it a bit more.
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https://code.launchpad.net/~sil2100/ubuntu-release-upgrader/snap-size-estimation/+merge/371120
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