improved workspace content subfolder for better test

jsonserver/static_db.json

     "content": [
       {
         "id": 1,
+        "parent_id": null,
+        "workspace_id": 1,
         "title": "La programmation fonctionnelle",
         "type": "PageHtml",
         "status": "validated"
       },
       {
         "id": 2,
+        "parent_id": null,
+        "workspace_id": 1,
         "title": "La programmation fonctionnelle est-elle vraiment utile ?",
         "type": "Thread",
         "status": "current"
       },
       {
         "id": 6,
+        "parent_id": null,
+        "workspace_id": 1,
         "title": "Une photo moche",
         "type": "File",
         "status": "current"
       },
       {
         "id": 10,
+        "parent_id": null,
+        "workspace_id": 1,
         "title": "le README.md d'un random repo github",
         "type": "PageMarkdown",
         "status": "current"
       },
       {
         "id": 7,
+        "parent_id": null,
+        "workspace_id": 1,
         "title": "Une liste de truc à faire que je ferai jamais",
         "type": "Task",
         "status": "current"
       },
       {
         "id": 8,
+        "parent_id": null,
+        "workspace_id": 1,
         "title": "Ça, ça marche pas. Merci de le fix",
         "type": "Issue",
         "status": "current"
       },
       {
         "id": 3,
+        "parent_id": null,
+        "workspace_id": 1,
         "title": "Programmation objet",
         "type": "folder",
         "content": [
           {
             "id": 4,
+            "parent_id": 3,
+            "workspace_id": 1,
             "title": "des trucs de backend",
             "type": "File",
             "status": "outdated"
           },
           {
             "id": 5,
+            "parent_id": 3,
+            "workspace_id": 1,
             "title": "on s'emmerde",
             "type": "PageHtml",
             "status": "outdated"
+          },
+          {
+            "id": 11,
+            "parent_id": 3,
+            "workspace_id": 1,
+            "title": "Hidden features",
+            "type": "folder",
+            "content": [
+              {
+                "id": 12,
+                "parent_id": 11,
+                "workspace_id": 1,
+                "title": "flatMap",
+                "type": "PageHtml",
+                "status": "current"
+              },
+              {
+                "id": 13,
+                "parent_id": 11,
+                "workspace_id": 1,
+                "title": "c'est tout pété",
+                "type": "Issue",
+                "status": "current"
+              }
+            ]
           }
         ]
       }