add raw git url for the avatar in the mockapi

jsonserver/static_db.json

     "is_active": true,
     "caldav_url": null,
     "user_id": 1,
-    "avatar_url": null,
+    "avatar_url": "https://rawgit.com/tracim/tracim_frontend/develop/src/img/adminProfil.png",
     "created": "2018-05-17T08:43:03.745219+00:00",
     "display_name": "Global manager"
   },