Supporting more Drupal forms

Lupus Decoupled Drupal supports submitting Drupal forms via the decoupled frontend, as documented under Advanced Topics - Drupal Forms. In order to enable submitting another Drupal form on the frontend, a custom-elements enabled form processing route must be added. This is done easily by:

1. Ensuring there is a custom-elements enabled route, e.g. by cloning the original one and adding custom_elements as _format.
2. Pointing the custom-elements enabled route to a form controller that renders the results as custom elements. For standard forms, the following controllers may be specified:

• For general forms: lupus_decoupled_form.controller.form:getContentResult
• For entity forms: lupus_decoupled_form.controller.entity_form:getContentResult
  Else, a custom controller can be easily be provided with the help of the CustomElementsFormControllerTrait, see the lupus_decoupled_contact submodule for an example.

That's it!

Generally, The Lupus Decoupled Contact module serves as a simple example that demonstrates how support for contact forms is enabled.