Intent to Desupport: Fennec Automated Updates (when Sideload Installed)

Hello,

I am intending to stop offering Fennec updates when Fennec itself is
installed manually instead of via the Google Play store.

More details are at https://github.com/mozilla-releng/releng-rfcs/pull/13
or in rendered form at
https://github.com/Callek/releng-rfcs/blob/no_fennec_balrog/rfcs/0013-disable-fennec-balrog.md

Please contain (most) comments to the markdown on github.

I intend to implement this on autoland/central as of Feb 5, 2019 and allow
it to ride the trains to release.

Thank You,
~Justin Wood (Callek)
Release Engineer