Differentiate between "nightly development config" and "release config"

[jdm]

Right now we have prefs.json, which is applied to local builds and nightly builds. There is also package-prefs.json, which is merged into prefs.json for nightly builds. However, in some cases we would like local and nightly builds to behave the same, but we also want some kind of config that applies to blessed builds of some kind that represent something that is intended for a wider audience. We should create a third json file that is merged in this case, and add a flag to ./mach package that indicates that this configuration is desired.

[highfive]

cc @wafflespeanut

[highfive]

cc @aneeshusa