Add extra layer of confirmation/security for switching a repo from private to public. (BB-12579)

Turning a private repo into a public repo is pretty easy - just uncheck a checkbox, hit save, and the repo becomes public. Maybe require the admin to put in the current password or another extra layer of security.

This is related to https://bitbucket.org/site/master/issue/8363/have-an-admin-switch-to-set-all-account.