Add and apply Rails/WhereNot rubocop rule

We simplify the code a little bit, and make it more consistent since we
were already using `where.not` in most places.

.rubocop.yml

@@ -325,6 +325,9 @@ Rails/UnknownEnv:
 Rails/Validation:
   Enabled: true
 
+Rails/WhereNot:
+  Enabled: true
+
 RSpec/AroundBlock:
   Enabled: true