Enable hiding and showing user needs by using a config initialiser

This adds a `InfoFrontend::FeatureFlags` module that we can use to set
different configuration per environment.

Using the feature flag attribute of `show_needs` we can hide and show
user needs by changing the value in this file.
Merge pull request #22 from alphagov/hide-needs-using-config-initialiser

Enable hiding and showing user needs by using a config initialiser