Add a basic background image to the homepage

Map screenshot from google maps, tweaked a little and arranged with
css to be a centered background image. A few minor tweaks to the
search box too, so that it appears to "float" in the map.

I also made the map on the search page 100% to mirror this look.
Allow ThemeChooser to be overriden by a config variable

Sometimes (e.g. for staging/test sites) it's easier to be
able to hard-code what theme the site should use, rather
than rely on the theme itself to correctly recognise the
url. This commit adds a config variable THEME which you
can set to force a particular theme to always be used.
Merge branch 'issues/4-map-background' into hampshire