Merge pull request #2 from alphagov/add_child_benefit_calc

Add child benefit calc
Merge pull request #3 from alphagov/add_child_benefit_calc

Add child benefit calc
Merge pull request #4 from alphagov/add_child_benefit_calc

Add child benefit calc
Tidy up view conditional
Add the new way starting children work

This completely changed so required a fairly large rework.
Remove old partial
Tweak how stopping children work
updated start page, updated copy, css changes
Merge Amy's CSS changes
Update specs to match new CSS and wording changes