Add and apply EmptyLineAfterGuardClause rule

We were inconsistent on this one. I consider it particularly useful when
a method starts with a `return` statement.

In other cases, we probably shouldn't have a guard rule in the middle of
a method in any case, but that's a different refactoring.

app/controllers/verification/sms_controller.rb

@@ -57,6 +57,7 @@ class Verification::SmsController < ApplicationController
 
     def verified_user
       return false unless params[:verified_user]
+
       @verified_user = VerifiedUser.by_user(current_user).where(id: params[:verified_user][:id]).first
     end