Fix datepicker

This at least makes it possible to test :)
I have a separate story to improve client side validation. We should capture any additional server side validation requirements as well.

Note that with this date picker the picker control activates with any focus of the element. When it was disabled that prevented all input.

I will test and try to induce some error scenarios.

> On Oct 13, 2014, at 2:55 PM, Victoria Earl <notifications@github.com> wrote:
> 
> In uber/templates/preregistration/age_and_staffing.html:
> 
> > @@ -25,7 +25,7 @@
> >  <label for="{% if COLLECT_EXACT_BIRTHDATE %}birthdate{% else %}age_group{% endif %}" class="col-sm-2 control-label">Age as of {{ EVENT_NAME_AND_YEAR }}:</label>
> >  <div class="col-sm-6">
> >  {% if COLLECT_EXACT_BIRTHDATE %}
> > -    <input type="text" class="form-control" name="birthdate" value="{{ attendee.birthdate|date:"Y-m-d"|default:"Click to select" }}" readonly="true" class="datepicker">
> > +                    <input type='text' class="form-control date" name="birthdate" value="{{ attendee.birthdate|date:"Y-m-d"|default:"Click to select"}} "/>
> Will this text box still be read-only? Model_checks.py doesn't have a lot of validation checks for bad data yet.
> 
> —
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