Modify public sign-up form to accommodate multiple projects

The shift selection can be removed from this form, as this will be handled on the public listing page now.

Presently this form takes a project ID as an argument via the URL and builds the list of profiles for the volunteer to fill out. This form should be modified to accept Need IDs instead, as the user will have specified these on the public listing UI, and the form should fetch the profiles for all the projects represented by those Needs.