[DO NOT MERGE] 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.
You should add the following tag to trigger this locally and CI:

```gherkin
@enable-pensions-calculator
Scenario Outline: View the Pensions Calculator
```