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/system_message_spec.rb

@@ -8,8 +8,8 @@ describe SystemMessage do
 
   context 'send' do
 
-    let(:admin) { Fabricate(:admin) }
-    let(:user) { Fabricate(:user) }
+    fab!(:admin) { Fabricate(:admin) }
+    fab!(:user) { Fabricate(:user) }
 
     before do
       SiteSetting.site_contact_username = admin.username