AJAX is broken on the add condition form

Problem/Motivation

It is impossible to use AJAX on the condition configuration form due to the way the add condition form is rendered by ManageConditions::add(). The form is rendered on a route where it doesn't exist and then displayed in a modal. It's also the reason why the form action has to be overwritten by the same method.

Steps to reproduce

• Install Entity Field Condition module from this MR or any other condition plugin with AJAX.
• Try to add this type of condition to any list.
• Trigger AJAX on the condition form. It does nothing.

Proposed resolution

In order for AJAX to work, the module has to make a sub-request to the form route. This way the form is rendered on a route where it exists and AJAX requests work as expected.

Remaining tasks

No.

User interface changes

No.

API changes

• \Drupal\ctools\Form\ManageConditions::getConditionClass() is deleted, as it's not necessary anymore.

Data model changes

No.