Create a separate controller and view for the main user interface

The whole form cart download system was squatting in housing_form's controller
and view.  Now, the housing_form controller and view have been reverted to more
standard rails scaffold style, and the shopping cart has its own place to live
in form_picker.
Merge pull request #35 from jrunningen/download-zip

Download zip, take two
Fix redirect urls for adding and removing forms from cart

Forgot to switch these over when I relocated the shopping cart to form_picker.