V4::Epic - Add `web_edit_url` and `reference`

ee/lib/ee/api/entities.rb

@@ -260,6 +260,12 @@ module EE
         expose :due_date_is_fixed?, as: :due_date_is_fixed, if: can_admin_epic
         expose :due_date_fixed, :due_date_from_milestones, if: can_admin_epic
         expose :state
+        expose :web_edit_url do |epic|
+          ::Gitlab::Routing.url_helpers.group_epic_path(epic.group, epic)
+        end
+        expose :reference do |epic|
+          epic.to_reference(epic.parent&.group)
+        end
         expose :created_at
         expose :updated_at
         expose :labels do |epic|

ee/spec/fixtures/api/schemas/public_api/v4/epic.json

@@ -38,7 +38,9 @@
     "due_date_is_fixed": { "type": "boolean" },
     "state": { "type": "string" },
     "created_at": { "type": ["string", "null"] },
-    "updated_at": { "type": ["string", "null"] }
+    "updated_at": { "type": ["string", "null"] },
+    "web_edit_url": { "type":  "string" },
+    "reference": { "type":  "string" }
   },
   "required": [
     "id", "iid", "group_id", "title"