Add an abstract environment 'defaults' in config for DRY

Fixes #249 #289.
Yes, but we want config-local to not contain a `root` and `base` key and for them to be derived automatically. But this way they will default to these values and they will need to be overwritten. I think these keys in particular should never be required to be specified in any config files – they are helpful to have for our application to function, but are not really part of the configuration formally.