Enable RSpec/DescribeClass rubocop rspec cop & fix issues

Manually fixes all issues & "stashed" false positives at .rubocop_todo.yml

Read about the cop at http://www.rubydoc.info/gems/rubocop-rspec/RuboCop/Cop/RSpec/DescribeClass

spec/models/abilities/organization_spec.rb

@@ -1,7 +1,7 @@
 require 'rails_helper'
 require 'cancan/matchers'
 
-describe "Abilities::Organization" do
+describe 'Abilities::Organization' do
   subject(:ability) { Ability.new(user) }
   let(:user) { organization.user }
   let(:organization) { create(:organization) }