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

[Łukasz Zemczak]

I did some testing on a VM and real upgrade scenarios and with this branch and the core18-stable follow up one, everything seems to work as expected. Let me merge both in this case.
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https://code.launchpad.net/~sil2100/ubuntu-release-upgrader/snap-size-estimation/+merge/371120
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