Ability to "Opt out" of upgrades.

[flexiondotorg]

Originally reported by: Luke Horwell (Bitbucket: lah7, GitHub: lah7)

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At the moment, once a user upgrades, the user cannot change their mind.

For instance, if a new version of LibreOffice caused problems [for the user's needs] and wishes to opt back to Ubuntu's repositories version, they can't do so via Welcome.

I propose we add an implementation to "undo" the changes (delete source file, roll back package version) if the user wishes to do so.

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• Bitbucket: https://bitbucket.org/ubuntu-mate/ubuntu-mate-welcome/issue/8

[flexiondotorg]

Original comment by lyzardking (Bitbucket: lyzardking, GitHub: lyzardking):

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This could be done using ppa-purge (installed only on the first downgrade select).
Is there any other way to remove the packages from a ppa properly?