Crop map

Added a map of where crops are being grown, as per https://www.pivotaltracker.com/story/show/53272283

Side effects/other stuff I did while working on this:

- I've rearranged the crop detail page to put more stuff in the sidebar (varieties, planting advice, etc)
- I changed the leaflet maps' clustering to cluster less vigorously, so that it shows more clusters with fewer members, because that has been bugging me on the main "places" map for ages 

Known bugs/weirdnesses:

- I don't seem to be able to get the map to zoom in a way that shows New Zealand (on my screen) yet isn't teeny tiny and repeating multiple times across the screen.  I think this is a leaflet problem -- there simply aren't any zoom steps between 1 and 2 and what we really want here is about 1.5 :-/

Potential future work:
- include a picture in the popup
- consider how/whether to show harvests as well