Migrate default systemd unit to templated unit
This allows admins to run several instances with different MIX_ENV
on same server, from same source directory.
There is no need to migrate to this for existing admins, because we only explicitly specify MIX_ENV=prod
here.
Was using this locally on my server for a while now to minimize maintenance efforts while hosting expired.mentality.rip, huskyapp.dev and tiny-pillowfort.floating-in.space. Works fine.
Newroma PR: https://codeberg.org/newroma-dev/newroma/pulls/47