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/lib/upload_creator_spec.rb

@@ -4,7 +4,7 @@ require 'rails_helper'
 require 'file_store/s3_store'
 
 RSpec.describe UploadCreator do
-  let(:user) { Fabricate(:user) }
+  fab!(:user) { Fabricate(:user) }
 
   describe '#create_for' do
     describe 'when upload is not an image' do