Added parties & categories to FTT views

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.

subcategory is already defined
Removed EAT & FTT helper & moved to GenericHelper

Moved display_parties helper to GenericHelper

Handling the missing (sub)categories in helpers

Streamlined the display_parties helper

Fix generic helper

Remove redundant whitespace

Simplify join display strings helper

Set default separator for display helper. Allow override from view.