Use rails helper `link_to` to render <a> elements for track links

This removes redundant leading whitespaces in a link text (in Firefox).
Fix track links to use href as audio src

This gives opportunity to download tracks easier or open it in a new
browser tab/window to listen to it via native browser player.
Merge pull request #2 from phts/fix-track-links-href

Fix track links to use href as audio src