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/direct_uploads_controller.rb

@@ -28,7 +28,13 @@ class DirectUploadsController < ApplicationController
 
     def direct_upload_params
       params.require(:direct_upload)
-            .permit(:resource, :resource_type, :resource_id, :resource_relation,
-                    :attachment, :cached_attachment, attachment_attributes: [])
+            .permit(allowed_params)
+    end
+
+    def allowed_params
+      [
+        :resource, :resource_type, :resource_id, :resource_relation,
+        :attachment, :cached_attachment, attachment_attributes: []
+      ]
     end
 end