Adds 'children' as an application field.

For camps that plan to have a kids' track, this adds a field to the
registration form for folks to indicate how many kids they plan to bring. The
data is not displayed publicly; It's only added for logistical purposes for
conference organizers.

The only thing I'm uncertain about is that the field is already checked under the Settings panel. Seems like it shouldn't be checked by default, but it also seems like making that possible is a significant change. If you all have thoughts on this, let me know.