Add summary entity to `FeatureFlagController#index` endpoint

What does this MR do?

This MR adds an endpoint which presents the summary of feature flags.

This is an answer to https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8821/diffs#note_129117025

An example of payload

{
   "feature_flags":[
      {
         "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"
      }
   ],
   "count":{
      "all":2,
      "enabled":1,
      "disabled":1
   }
}

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