Move httpserver repository from qt-labs to qt-extensions

As agreed on the mailing list, the project:qt-labs/qthttpserver should move out of the qt-labs namespace and instead be located at something indicating a stable version.

The easiest would be to move it to qt-apps/qthttpserver, as that is an existing namespace.

Another option would be to introduce a new one, but I am not sure about the process here.