Provide generic DateHelper module containing next_saturday method

This is the start of the work to reduce the duplication of the
next_saturday code that currently exists.
Use new DateHelper module

This removes the duplication of the next_saturday methods in the lib/
and test/ directories.
Merge remote branch 'origin/maternity-paternity-content-and-calculator-fixes'

Conflicts:
	lib/smart_answer/date_helper.rb
	test/unit/date_helper_test.rb