Correct flow for people born after 1992-04-05

If born after this date, you should be asked how many years of work you did when aged 16-19, instead of going directly to the results page.
Correctly calculate automatic credits in Q6

Previously, the automatic credit calculation was delayed until Q10.
Meaning Q11 was potentially needlessly asked.
Fix leap year calculation - days between two dates

There is no need to remove a day from whole_days_away. Ruby / Rails already gives the correct calculation.
Merge pull request #1179 from karlentwistle/master

Fix bugs in state pension calculator V2

As discussed with @karlentwhistle future PRs will be on branches from alphagov/smart-answers. I've checked this fork out and tests all pass.