Flatten the project at export and support import for Linux

The client will send a flag to say if the import should be done
on the VM or not

Ref https://github.com/GNS3/gns3-gui/issues/1151

gns3server/handlers/api/project_handler.py

@@ -405,7 +405,7 @@ class ProjectHandler:
                     if not packet:
                         break
                     temp.write(packet)
-                project.import_zip(temp)
+                project.import_zip(temp, gns3vm=bool(request.GET.get("gns3vm", "1")))
         except OSError as e:
             raise aiohttp.web.HTTPInternalServerError(text="Could not import the project: {}".format(e))