Fix account links in celerity

* moves common formatting of nav to htdocs/scss/schemes/_nav.scss
Make the celerity menus responsive
Expand the width the acount links can take up
Fix display issues at bottom of celerity menu

Two reasons:
* unclosed div
* position: relative messing with how it interacts with other elements
  at the bottom of the page
Make sure we apply clearing to #content no matter the screen size

Was previously within a media query, which meant that thinsg grew wonky
at particular screen sizes
Fix search bar in celerity
No more need for IE6 hack

Rest of our frontend no longer supports IE6, so this wouldn't make a big
difference if it disappeared.