no longer present the option to the user of signing up for conflicting shifts.  instead, pre filter the individual shifts (AM/PM and day) to not overlap what they've picked.  this means the signup form has to send the email address back to the server before it can come back with a list of available intersections and give the user an error if they try to look at an intersection without giving an email address first.  it sends various data fields back to the server on blur now, related to that.