Show most popular crops on "browse crops" page

https://www.pivotaltracker.com/story/show/37282743

When you go to "Browse crops" it now defaults to showing the most popular crops, rather than alphabetical order (but it falls back to alphabetical if nothing's been planted, or for multiple crops that have been planted the same number of times.)

There is also a form at the top of the page letting you choose your preferred sort order.