Use separate accounts to store the different keys.

Update Mullvad Browser release prep templates to include downstream package notification/maintenance

This commit adds a config variable to override the URL for the updater.
When it is defined, a user-provided certificate will replace the
default public keys used to sign nightly builds.

Also, while we are at it, switch the git repo to HTTPS.

We also update doc/MAKEFILE.txt for previous changes with some makefile
targets.