Changed and improved signups for demo users.

Demo accounts used to be created when visitors clicked one of the available checklists. The demo account was created, the checklist was added to it, and the user was redirected back to the front page. This process now supports redirects via adding a "redirect_to" field to the form, such as:

    <input type="hidden" name="redirect_to" value={{ document.urlname }}>

This is especially useful if you want to present users with a single "Start a demo!" button instead of a list of checklists, because the user can be redirected immediately to the demo checklist after clicking the button.