Split aero and integrant into two separate parts.

[bryanmikaelian]

The integrant + aero config map is very confusing. It took me at least a week to decrypt what was done and how things even functioned. This was originally copy-pasta from kit. It makes sense for that framework but not for our needs.

Instead, we are just following the suggestions in this blog post and decoupling the two.

1. aero loads system.edn, interpreting env vars when appropriate
2. We merge that with a settings map filled with ig/refs
3. system/start gets called, calling ig/prep and ig/init under the hood.