Toggling between maps for multiple cultural / environmental variables, map is downloadable (sort of?)

Fix for #168 , #45 in progress

Current position of legend will be changed.

The map is downloadable, however, at the moment users need to visit the map tab before the map download link is appended to the 'downloads' list.  At the moment, if multiple traits are selected, the relevant map downloads are appended to the 'downloads' list one by one as they are selected.  This was just quick code to see if I could get it working.

- What to display for society markers on society results table?  
- Still need to add a legend for environmental variables

Hmm. There is when I run it. it works just like the rest of the `ReadOnlyModelViewSet`s