Add and apply Rails/HttpStatus rubocop rule

We were already using it in most places. Since rubocop-rails 2.11.0,
this rule also detects offenses when using the `head` method, which we
were using with a plain `404`.

.rubocop.yml

@@ -245,6 +245,9 @@ Rails/HasManyOrHasOneDependent:
   Enabled: true
   Severity: refactor
 
+Rails/HttpStatus:
+  Enabled: true
+
 Rails/InverseOf:
   Enabled: true
   Exclude: