PT: https://www.pivotaltracker.com/story/show/51457917
Maco found this. The problem was that if you had a blank planting date,
and then re-edited the planting, it would say "not yet set" in the form
field, then die when it later tried to convert that to a date.
I replaced Miles's planted_at_string stuff in the model with a simpler
parse_date method in the application helper.
I tried to do this using Haml::Filters::Markdown so it would be
independent of our Markdown renderer, but I couldn't get
Haml::Filters::Markdown.render to work programmatically - whatever I
did, it returned its input unchanged. So I hardcoded a dependence on
bluecloth.
