Add per-environment view to edit feature flags

Problem to solve

https://gitlab.com/gitlab-org/gitlab-ee/issues/8621 introduces a feature-flag oriented view to edit feature flags. Looking at a feature flag and seeing what the values are in all the environments for that flag is an important view, but there is also an important context where you want to look at one environment and see what feature flags are set for it.

This issue adds the per-environment view for managing feature flags.

Target audience

Feature flag managers

Further details

TBD

Proposal

TBD (depends partially on approach determined in https://gitlab.com/gitlab-org/gitlab-ee/issues/8621 for per-flag context.

What does success look like, and how can we measure that?

TBD