Replace download <button> by <a>

Hi,

this is a really small thing, but when you set a seafile document/directory as public and share it with a link, people have to click the link then click the "Download" button to actually get the file(s).

What's wrong, in my opinion, is that the "Download" button is not a link. So I can't share it directly with people. As far as I understand, clicking on this button call some javascript, which trigger the download process.

If I'm right, it means search-engine and bots that don't enable javascript can't accesss shared documents. 

So, I've made this pull request to simply replace the <button> tah by an <a>.