move: Add support for moving backport packages.

This replaces two scripts found on the build system: mga-move-backports-{nosync,sync-mail}

The latter scripts name includes the word 'sync' but it didn't actually do the sync.
but as this script supports a --no-sync option, this is now fully optional.

There seemed to be a stray $ in the old file. I don't think it was deliberate.

The mail sent to the backports ML is nicer than before as it doesn't include
output from raw commands, just a human readable equiv.

This also still sends a mail for audit purposes regardless of the nice
announce email

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