Make it harder to push to release

[tombettany]

Extend the push command to ask for confirmation when pushing to the stable suite (similar to what apt-get install pkg does).

What needs to be done

1. Change the push subcommand here to ask for confirmation if the suite is stable.
2. Change the dr build <pkg> -p <suite> shortcut to pushing to ask for the same confirmation when pushing to stable.
3. Implement a similar confirmation for dr unpush subcommand from stable.

[pazdera]

I like that idea. We could have something similar apt: y/[n] question and --yes to don't even ask. Or we could do the same GitHub does when you delete repositories: type the name of the package you're about to push.

:+1:

[convolu]

:+1: