added help text/links to planting form

Two 1-point stories in one pull request:

https://www.pivotaltracker.com/story/show/56022062
https://www.pivotaltracker.com/story/show/56022170

At the request of one of our new members, we're adding some explanatory/helpful text and links to the planting form:

# a link to the "request new crops" forum thread next to the crop dropdown (we made this a config variable, as it's meaningless anywhere except production)
# a link to "add a garden" next to the garden dropdown