A set of classes to encapsulate details for mounting engines.

Setup for our first engine, pensions calculator, with a pattern that can be expanded for other engines.
Implement the language translation link and alternate meta tags for engines.
Implement correct constraints for the pensions calculator.
Merge pull request #626 from moneyadviceservice/pc-locale-switch

Locale change link and alternate meta tags for engines / pensions calc
Don’t show footer opt out link for feedback pages.