Make dev and staging libraries default to upstream

[#1]

Currently, if there is no library assigned to a dev or staging environment, it 404s. This can (reasonably) bother developers and break analytics testing on dev/staging/QA servers.

Ideally, if an environment does NOT have a library currently on it, it would just use whatever is currently upstream in prod.

[#2]

Adding an option to allow enable/disable upstream would be helpful as well.

[#3]

We had some Akamai problems in the past that led to Dev Libraries being removed when they shouldn't be, but that has been fixed now.  The only time an Environment should ever return a 404 now is when it is first created and has never been built in.  And defaulting to Prod often wouldn't help in that scenario because it wouldn't have anything in it either.

One idea that has been floated is that when an Environment is created, we just drop an empty build there so that it wouldn't 404.  Curious to hear people's thoughts.

[#4]

I think that'd be great, as a bare minimum.

[#5]

Building empty libraries for new environments is on the roadmap for this year.