Apply the 6 April 2017 child cutoffs by date of birth, not birth year #1903

Four rules turn on whether a child was born before 6 April 2017:

• the Universal Credit child limit (UC Regs 2013 reg 24A(3), in force 6 April 2017 to 5 April 2026);
• the higher UC child element for the first child (UC (Transitional Provisions) Regs 2014 reg 43);
• the Child Tax Credit child limit (Tax Credits Act 2002 s.9(3A));
• the Pension Credit first child addition (SPC Regs 2002 Sch IIA para 10).

The model compares birth_year (period less whole age) with a year: uc_is_child_born_before_child_limit (birth_year < start_year), is_CTC_child_limit_exempt, and child_minimum_guarantee_addition (a hard-coded birth_year < 2017). With a whole age, every child aged 9 in 2026-27 counts as born in 2017, whether born in January (before the cutoff) or May (after it). In survey data, the whole cohort born from October 2016 to October 2017 is treated as born after the cutoff, when about half of it was born before.

#1899 adds months_since_last_birthday, which places each person's date of birth within their year of age: 6 for household situations, spread evenly by weight in data. The fix is to:

• derive a date of birth from it;
• compare that with 6 April 2017, held as a date parameter;
• order UC and Pension Credit children by date of birth (UC Regs reg 24B; Sch IIA para 10's "eldest"), so the first child is the one born first;
• derive birth_year from the date of birth.