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/components/admin_confirmation_spec.rb

@@ -5,8 +5,8 @@ require 'rails_helper'
 
 describe AdminConfirmation do
 
-  let(:admin) { Fabricate(:admin) }
-  let(:user) { Fabricate(:user) }
+  fab!(:admin) { Fabricate(:admin) }
+  fab!(:user) { Fabricate(:user) }
 
   describe "create_confirmation" do
     it "raises an error for non-admins" do