tahoe.css: fix #section typo, update welcome.xhtml to match

tahoe.css: fix #section typo, update welcome.xhtml to match

The "#section" declaration (which matches id="section") should have been
".section" (which matches class="section").

The welcome page has a feature that I actually liked: the little "This
Client" sidebar sits just to the right of the start of the Controls block.
Fixing .section broke that (the clear:both introduces a gap, forcing the
Controls block to start strictly below the bottom of the This Client block).
So I also removed class="section" from the Controls block to allow them to
share the horizontal space again.