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.
erb files are just erb files, and their *usage* determines whether they're operating as a partial/template/layout etc.

as it stands, https://github.com/moneyadviceservice/frontend/blob/master/app/views/layouts/constrained.html.erb, would work just find with the new approach.