New fields for gardens

This pull request includes work on three different stories, which were all so similar that it seemed like a good idea to do them together.

1) Mark gardens as inactive https://www.pivotaltracker.com/story/show/53272391
2) Track area/size of gardens https://www.pivotaltracker.com/story/show/57111618
3) Specify garden location https://www.pivotaltracker.com/story/show/57111662

This includes a rake task to initialize any garden locations to the owner's location if the owner's location is known.

We've spawned another story (https://www.pivotaltracker.com/story/show/59555864) to display inactive gardens differently in various places, but since most of those pages are due for a redesign anyway, we figured it could be done then.