Add active categories to action plans

This allows highlight the current category in te left hand navigation
This could possibly be moved into the ``Navigation`` concern seeing as something similar is happening [here](https://github.com/moneyadviceservice/frontend/blob/bug/action-plan-lef-navigation/app/controllers/articles_controller.rb#L16) and [here](https://github.com/moneyadviceservice/frontend/blob/bug/action-plan-lef-navigation/app/controllers/categories_controller.rb#L13)