Mount PensionsCalculator (behind a feature flag)

* introduces bowndler for bower/bundler integration
* references pensions_calculator branch on github until
  pensions_calculator released gem is ready to be mounted
Introduce embedded_tool layout for housing engines.
Add mechanism for pulling in an engine’s js via requirejs.
Move bowndler to development group
Various changes to layout integration with the pensions_calculator engine.
Pull RequireJS config into layout not into application.js
Load require config for engine before its main module
Fix incorrect welsh id in routes.
Add smoke test for the pension calculator.

Note that this currently fails due to lack of welsh translations and dependency.

It is also dependent on a feature flag being on.
Switch to an officially released pensions calculator gem.
Merge branch 'mount_pensions_calculator'