Add JSON responder at `#index` endpoint in feature flag controller

What does this MR do?

This MR adds JSON responder to FeatureFlagController#index endpoint. It represents entities as the following schema.

[
   {
      "id":1,
      "active":true,
      "created_at":"2019-01-03T09:34:02.337Z",
      "updated_at":"2019-01-03T09:34:02.337Z",
      "name":"test",
      "description":"aaaaaaaaaa",
      "edit_path":"/root/feature-flag-json-responder/-/feature_flags/1/edit",
      "destroy_path":"/root/feature-flag-json-responder/-/feature_flags/1"
   },
   {
      "id":2,
      "active":false,
      "created_at":"2019-01-03T09:34:10.873Z",
      "updated_at":"2019-01-03T09:34:10.873Z",
      "name":"test-2",
      "description":"bbbbbb",
      "edit_path":"/root/feature-flag-json-responder/-/feature_flags/2/edit",
      "destroy_path":"/root/feature-flag-json-responder/-/feature_flags/2"
   }
]

This will be used in https://gitlab.com/gitlab-org/gitlab-ee/issues/7731. Frontend part is worked in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8821#note_126139249 by @shampton .

What are the relevant issue numbers?

Related: https://gitlab.com/gitlab-org/gitlab-ee/issues/7731

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