DEV: Prefabrication (test optimization) (#7414)

* Introduced fab!, a helper that creates database state for a group

It's almost identical to let_it_be, except:

 1. It creates a new object for each test by default,
 2. You can disable it using PREFABRICATION=0

spec/helpers/application_helper_spec.rb

@@ -166,7 +166,7 @@ describe ApplicationHelper do
   end
 
   describe '#html_classes' do
-    let(:user) { Fabricate(:user) }
+    fab!(:user) { Fabricate(:user) }
 
     it "includes 'rtl' when the I18n.locale is rtl" do
       I18n.stubs(:locale).returns(:he)