Adds environment support
This PR adds environments as a first-class concept in Unleash.
It necessitated a full rewrite on how we connect feature <-> strategy, as well as a rethink on which levels environments makes sense.
This enables PUTs on strategy configurations for a feature, since all strategies now have ids.
This also updates export/import format. The importer handles both formats, but export is no longer possible in version 1 of the export format, only in version 2, with strategy configurations for a feature as a separate object.
Co-authored-by: Christopher Kolstad <chriswk@getunleash.ai>
Co-authored-by: Fredrik Oseberg <fredrik.no@gmail.com>
Co-authored-by: Ivar Conradi Østhus <ivarconr@gmail.com>
