ui-shared: pass separator in to cgit_print_snapshot_links()

cgit_print_snapshot_links() is almost identical to
print_tag_downloads(), so let's extract the difference to a parameter in
preparation for removing print_tag_downloads() in the next commit.

Signed-off-by: John Keeping <john@keeping.me.uk>
Reviewed-by: Christian Hesse <mail@eworm.de>