Redesign

This pull request tracks work on a redesign of the header, sidebar, and main page map based on the presentation ([video](http://osm.won2.de/sotm/d2t1-1630-John_Firebaugh-Refining_a_vision_for_OpenStreetMap.org.mp4), [slides](http://jfire.io/sotm13/), [mock ups](https://github.com/samanpwbb/osm-redesign/tree/sotm-birmingham/renders)) at State of the Map Birmingham.

Major components of the redesign include a new help page; new about page; moving navigation elements out of the sidebar; moving search results, export controls, changeset history, and feature details into the sidebar; and removal of the sidebar from "interior" pages such as the user profile and diaries.

## TODO List

* [ ] Finalize header design
* [ ] Add call to action button to main sidebar
* [ ] Implement closing the sidebar
* [ ] Hook up interactivity for changeset view (load changesets for current view, update on map move)
* [ ] pushState-based navigation
* [ ] Restore sidebar when editing with Potlatch