PoC: per environment feature flag

What does this MR do?

Creation/Edit form

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• Environments
  • production
  • staging
  • review/feature-1
  • review/feature-2

---

Case 1:

• Feature flags
  • Name: flag-1, Environment scope: "*", Active: true

Result: 1

• Environments
  • production => flag-1 is true
  • staging => flag-1 is true
  • review/feature-1 => flag-1 is true
  • review/feature-2 => flag-1 is true

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Case 2:

• Feature flags
  • Name: flag-1, Environment scope: "*", Active: false
  • Name: flag-1, Environment scope: "review/*", Active: true

Result: 2

• Environments
  • production => flag-1 is false
  • staging => flag-1 is false
  • review/feature-1 => flag-1 is true
  • review/feature-2 => flag-1 is true

---

Case 3:

• Feature flags
  • Name: flag-1, Environment scope: "*", Active: true
  • Name: flag-1, Environment scope: "production", Active: false

Result: 3

• Environments
  • production => flag-1 is false
  • staging => flag-1 is true
  • review/feature-1 => flag-1 is true
  • review/feature-2 => flag-1 is true

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Configuration form

Index page (http://localhost:8181/root/feature-flag-json-responder/-/feature_flags)

Environment page

Environment specific Index page (http://localhost:8181/root/feature-flag-json-responder/-/feature_flags?environmnet_name=production)

What are the relevant issue numbers?

Related: https://gitlab.com/gitlab-org/gitlab-ee/issues/8621

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer