Finished plantings

PLEASE REVIEW BUT DO NOT MERGE. (We're about to do a production push.)

This PR allows you to mark plantings as "finished", as per https://www.pivotaltracker.com/story/show/46972853

Along the way I also tweaked gardens a little bit to give them a handy "mark as inactive" button, and to mark all plantings in the garden as finished when a garden is inactive.
Can we also test that if there's another garden owned by the same person, plantings in that one don't get marked as finished? That's the kind of logic that would be easy to break by someone changing things in the future.