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/requests/static_controller_spec.rb

@@ -3,7 +3,7 @@
 require 'rails_helper'
 
 describe StaticController do
-  let(:upload) { Fabricate(:upload) }
+  fab!(:upload) { Fabricate(:upload) }
 
   context '#favicon' do
     let(:filename) { 'smallest.png' }