Conform prosilver templates to 3.1

Fixes https://github.com/rxu/thanks_for_posts/issues/8 and https://github.com/rxu/thanks_for_posts/issues/9 (although not the `<a title="Thanks Toplist &mdash; %d" href="./toplist">` title problem.)

Also: `prosilver/memberlist_view_thanks.html` seems to never be used. Perhaps you can remove it.