Add Rails/UniqueValidationWithoutIndex rule

This is something we should do everywhere because concurrent requests
might bypass Rails uniqueness validations. However, since there are
places where we aren't applying this rule and adding a unique index
means we also need to destroy any hypothetical duplicate records, it's
something we aren't going to solve right now. Therefore we use the
"refactor" severity so existing offenses don't get in our way.

In any case, we're adding the rule so we don't make the same mistake in
the future.

.rubocop.yml

@@ -300,6 +300,10 @@ Rails/SkipsModelValidations:
 Rails/TimeZone:
   Enabled: true
 
+Rails/UniqueValidationWithoutIndex:
+  Enabled: true
+  Severity: refactor
+
 Rails/UnknownEnv:
   Enabled: true
   Environments: