Improve environment document caching behaviour to expire based on changes

[matthewelwell]

Currently the application has a configurable cache for environment documents (used by the environment-document endpoint in the Core API). This cache relies on a timeout.

We should improve this behaviour to support a centralised cache (e.g. redis, but maybe postgres by default), and we can invalidate the cache using the same trigger that builds the environment document for the Edge API for our SaaS environment.