Planting links

Now showing who's planted what, where.  Plantings show up on member, crops, and garden pages.

This is done via a planting thumbnail which is functional but not at all pretty, as per discussion with jcaudle last night -- we tried to make a pretty one like the mockup I drew, but the CSS was too hard.  This works even if not particularly attractive.

Something to look at: there is a method on the garden model for "featured plantings" -- it picks the most recent 4 plantings for that garden that don't repeat a crop, so that eg. Tomato isn't shown twice. The algorithm for this is kind of clunky, and I'm sure there's a more idiomatic way to do this, but we couldn't figure it out so, as with the thumbnails, we went with "works, but isn't real pretty".