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.

src/allmydata/web/static/tahoe.css

@@ -80,7 +80,7 @@ th.nickname-and-peerid {
 
 
 /* just in case, make sure floats don't stomp on big tables etc. */
-#section { clear: both; }
+.section { clear: both; }
 
 /* section-specific styles - turn this client info into a sidebar */
 #this-client {

src/allmydata/web/welcome.xhtml

@@ -22,11 +22,9 @@
     <tr><th>Tahoe-LAFS code imported from:</th> <td n:render="string" n:data="import_path" /></tr>
     <tr><th>Services running:</th> <td n:render="services" /></tr>
   </table>
-  
-
 </div>
 
-<div class="section" id="controls">
+<div id="controls">
 <h2>Controls</h2>
 
   <p>There are also controls for each directory on that directory's page.</p>