Cleanup the user interface for Feature Flag Strategies

Release notes

Problem to solve

Feature flag strategies are a powerful way for users to be able to determine progressive rollout of features, defining who does and does not see a feature. The strategies section of the UI doesn't match the GitLab standards and lowers the overall level of trust users have in a feature that defines if users can or cannot see something.

Proposal

Cleanup the user interface of the Feature Flag Strategies tab to align with newer GitLab standards and improved UX.

Proposed design