exposure/templates/exposure/export.html:

Fixed a static image reference.
exposure/static/exposure/exposure_export.js:

Rebased the exposure app on the new leaflet app template.
static/css:

Some minor tweaks to leaflet app styling (margins and bg).
templates/leaflet_base.html:

Include oqplatform leaflet-specific styling by default in the template.
static/js/base_leaflet_app.js:

The base leaflet app declares a `var map` in the global namespace,
which is needed for things like wax to work correctly.