DEV: Allow fab! without block (#24314)

The most common thing that we do with fab! is:

    fab!(:thing) { Fabricate(:thing) }

This commit adds a shorthand for this which is just simply:

    fab!(:thing)

i.e. If you omit the block, then, by default, you'll get a `Fabricate`d object using the fabricator of the same name.

spec/requests/do_not_disturb_controller_spec.rb

@@ -7,7 +7,7 @@ RSpec.describe DoNotDisturbController do
   end
 
   describe "logged in" do
-    fab!(:user) { Fabricate(:user) }
+    fab!(:user)
 
     before { sign_in(user) }