Added parties & categories to FTT views

**NOTE**: I'll sqash this before rebasing, but wanted to keep the commits separate because:

1. This was really the goal of this PR 

  Added parties & categories to FTT views, 

2. But this had to be done for it to work:

   Redesigned display_categories helper …

  Since all the helpers are available in all the views and I've reused the
  display_categories helper from EatDecisionsHelper in FttDecisionsHelper,
  when the examples were run, the helper from EatDecisionsHelper were
  being picked up first so the examples were failing.

  That's why I created the generic_helper.rb and added the helper method
  there, so that both models can use it.

  The only drawback of the method is that it will fail for non CamelCase
models such as Decision, whereas EatDecision is fine.

3. display_categories now handles three possible cases regarding categories & subcategories, 
see: https://github.com/ministryofjustice/tribunals/commit/9cf2306574e7b71544da63c23ce93a4cc170036d#diff-ec5be3b8264de9adb8fc70a1ffaf51e3R16