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.