Update feature flag UI to support percentage rollout
What does this MR do?
Updates the new/edit feature flag page (/user/project/-/feature_flags/new and /user/project/-/feature_flags/<id>/edit) and the main feature flag page (/user/project/-/feature_flags) to support the percentage rollout strategy.
In addition, this merge request does some non-trivial cleanup/refactoring of ee/app/assets/javascripts/feature_flags/components/form.vue.
Mockups:
New/edit page:
Main page:
This MR has a CE backport
gitlab-org/gitlab-ce!31240
Link to Demo
This merge request completes the frontend portions of #8240 (closed).
Edited by Orit Golowinski