renamed filename for downloading empty wiki on tw5.com

This changes the filename when clicking the `Download Empty` button to `tiddlywiki-$(version)$-empty.html`. I tested this in Firefox as well as chrome and it seems to work okay. This fixes #1234.


Note: This does not fix the `cryptic filename` bug (see #1231) and it does not change the download filenames of the `Download full wiki` button or any other button for that matter. Also note that the macro is not reusable and should not be put into the general macro tiddlers.

/Andreas