Make it easier to customize allowed parameters

When customizing CONSUL, one of the most common actions is adding a new
field to a form.

This requires modifying the permitted/allowed parameters. However, in
most cases, the method returning these parameters returned an instance
of `ActionController::Parameters`, so adding more parameters to it
wasn't easy.

So customizing the code required copying the method returning those
parameters and adding the new ones. For example:

```
def something_params
  params.require(:something).permit(
    :one_consul_attribute,
    :another_consul_attribute,
    :my_custom_attribute
  )
end
```

This meant that, if the `something_params` method changed in CONSUL, the
customization of this method had to be updated as well.

So we're extracting the logic returning the parameters to a method which
returns an array. Now this code can be customized without copying the
original method:

```
alias_method :consul_allowed_params, :allowed_params

def allowed_params
  consul_allowed_params + [:my_custom_attribute]
end
```

app/controllers/verification/letter_controller.rb

@@ -40,7 +40,11 @@ class Verification::LetterController < ApplicationController
   private
 
     def letter_params
-      params.require(:verification_letter).permit(:verification_code, :email, :password)
+      params.require(:verification_letter).permit(allowed_params)
+    end
+
+    def allowed_params
+      [:verification_code, :email, :password]
     end
 
     def verify_phone!