workspace user: use wapi function instead of creating object

tracim/views/core_api/user_controller.py

         
         workspaces = wapi.get_all_for_user(request.candidate_user)
         return [
-            WorkspaceInContext(workspace, request.dbsession, app_config)
+            wapi.get_workspace_with_context(workspace)
             for workspace in workspaces
         ]