Automate new stable releases

The creation of new stable releases is, in practice, currently a single-point-of-failure, where if I (@duncanmmacleod) don't do it, it doesn't get done. This is nobody's fault (except mine), but we should consider automating the creation of new stable releases.

To match up with traditional LIGO Lab engineering practice, my best guess would be to automate a new release for 8am Pacific time on a Tuesday morning, which should then work it's way through and present new usable stable environments in CVMFS by roughly 12pm Pacific time the same day. This can be done in gitlab ci using (a variant of) the scripts/release.sh that is used manually now.

@rhys.poulton @robert.bruntz @stuart.anderson @jameson.rollins (conda maintainers and major stakeholders), please comment.