4733 Mobility layout

Branchandroot's Mobility layout, plus two colour themes and preview images. 

The layout has been thoroughly tested, see http://dreamscapes.dreamwidth.org/115076.html 

NB Mobility doesn't inherit from Tabula Rasa and has a custom version of Page::print() which makes the layout work on most mobile devices as well as full-sized screens. 

Style review much appreciated (it's my first full layout under the current regime).