Provide secrets/passwords management for Tor Browser Nightly signing

As mentioned in legacy/trac#34121 (moved), the Tor Browser Nightly signing machine will host an OpenPGP key and an NSSDB private key. Both of these should be password-protected. Instead of hard-coding these passphrases in a file or script on the server, having a password management system from where the passwords can be retrieved would be very nice.

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