Production push: finished plantings, crowdfunding banner, etc

- Finished plantings: you can now mark a planting as "finished" and specify a date on which this happened
- Added a sitewide banner pointing at our crowdfunding campaign
- Fixed mailing list link to point to new Growstuff Talk forums
- Added placeholder text to give a hint re: crops on planting form etc
- Bugfix: if you click on "plant this" from a crop page, the crop field is now prefilled