Allow users to remove themselves from a package

[jonasfj]

I discussed this with @mit-mit today, once we have a history: https://github.com/dart-lang/pub-dev/issues/2687 it makes sense to allow users to removed themselves.

[jonasfj]

This is blocked by https://github.com/dart-lang/pub-dev/issues/2687.

[sigurdm]

Probably no longer blocked.

Can the last uploader remove itself?

Can we make this extra hard to do by mistake? eg. by requiring writing something to confirm.