Remove the use of resource controller instance variables for passing things down to the view

[jensljungblad]

While using instance variables for this is a Rails thing, we're not big fans of this. We're thinking about using locals instead, if possible.

Currently we have all these methods that only sets an instance variable:

before_action :set_resource_service
before_action :set_resource_class
before_action :set_resource_parents
before_action :set_resources, only: :index
before_action :set_resource, only: [:show, :new, :edit, :create, :update, :destroy]

Look into if it's possible to do something like:

def index
  locals = index_locals
  
  respond_to do |format|
    format.html { locals: locals }
    format.json { locals: locals }
    format.csv { locals: locals }
  end
end

protected

def index_locals
  { 
    resource_service: resource_service,
    resource_class: resource_class,
    resources: resources 
  }
end

Discussion regarding using locals: thoughtbot/guides#398, https://thepugautomatic.com/2013/05/locals/