add append date functionality for planting finished on garden show page ...

Implemented [this request](http://talk.growstuff.org/t/please-test-internationalisation-finished-plantings-javascript/116/3?u=tygriffin) made by juliet on Growstuff Talk:

"works correctly from the planting page. However, there are also 'mark as finished' buttons on the Garden page (e.g. http://dev.growstuff.org/gardens/juliet-back-garden) and the Planting list page (e.g. http://dev.growstuff.org/plantings/owner/juliet). No calendar pops up on either of those buttons (so it is recorded as 'no date set'); the calendar should pop up here too."