Fixed #647: UI discrepency in playlist tracks count

Closes #647 (closed)

@floreal: there actually was a discrepency in the value displayed in the playlist modal, which used the total number of elements in the playlist, and the value displayed when viewing the playlist page, which used the number of playable tracks.

Both value may differ, since files may be removed (and tracks become unplayable for a given user), but we don't reflect that change in playlist (since files may be reuploaded later ;)

At least, we should have a consistent UI now.