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/helpers/application_helper.rb

@@ -1,6 +1,7 @@
 module ApplicationHelper
   def home_page?
     return false if user_signed_in?
+
     # Using path because fullpath yields false negatives since it contains
     # parameters too
     request.path == "/"
@@ -41,6 +42,7 @@ module ApplicationHelper
 
   def author_of?(authorable, user)
     return false if authorable.blank? || user.blank?
+
     authorable.author_id == user.id
   end