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

This PR should only be merged once the pensions_calculator is hooked in from a built gem.
What you've described there _is_ the new method, isn't it?  But that would require ``layouts/constrained`` to be a partial.

In our situation both ``constrained_with_engine`` and ``constrained`` are non partial layouts so we have to revert back to the usual clunky approach of using ``content_for``.