Replace date_of_birth with birth_date

Thanks to Chris Mytton (@hecticjeff) for pointing out that in the Popolo
spec birth dates are birth_date, not date_of_birth.
Add Friday afternoon workarounds for the birth_date issue

This should more properly be fixed by one or both of:

 (a) PopIt treating empty string dates the same as null
     https://github.com/mysociety/popit-api/issues/98

 (b) Tracing back in this code to where we're setting
     dates to '', and change that to null.