Finish plantings javascript

@Marlena and I paired on a couple of stories related to marking a planting finished.

+ [First story](https://www.pivotaltracker.com/n/projects/646869/stories/78752894): Clear out the `finished_at` input field when marking a planting finished. The date is cached (as long as the user is on the page) in case the planting is re-marked finished. 

+ [Second story](https://www.pivotaltracker.com/n/projects/646869/stories/78753350): Show a datepicker to indicate the `finished_at` date when marking a planting finished on the planting show page.

For the first story we thought about auto-populating the `finished_at` input field with some arbitrary date like today's date when the planting is marked finished. But then we thought this might be unnecessarily complicated, so we're throwing it out there for general discussion.