Consolidate apt and yum repos onto one hostname

Let's merge:

• apt.freedom.press -> packages.securedrop.org/debian
• yum.securedrop.org-> packages.securedrop.org/dom0
  same for packages-test and packages-qa.

And then on the Git level:

• securedrop-apt-test + securedrop-yum-test -> securedrop-packages-test
• securedrop-apt-prod + securedrop-yum-prod -> securedrop-packages-prod

This somewhat the setup used by Dangerzone at https://packages.freedom.press/. It's cleaner and cuts down on some duplication, but the benefits are pretty minor, especially given we'd need to support the old hostnames for a while and migrate existing users over.

This was previously discussed by the team during the package repository redo work and was deferred. This issue is primarily to track the potential work outside of a wiki page :)

Start by reviewing the existing apt.freedom.press, yum.securedrop.org, packages-test, and packages-qa hostnames, along with the securedrop-apt-test/prod and securedrop-yum-test/prod repositories. Confirm the migration and backward-compatibility requirements with the prior package repository redo discussion; done would mean the repositories are consolidated while existing users can migrate from the old hostnames safely.