Close #187. Add Garden Model, simplify Sign Up flow

# What's changed.
* Gave each user a has_many gardens relationship.
* Moved a number of "gardeny" things out of the user model and into the garden model.
* Removed several fields from sign up form, designed sign up form.

# What's next
* A page for adding details (will probably involve Angular)
* A page for adding a garden (will probably also involve Angular)