Email body policy form should show available TWIG variables

Problem/Motivation

The description for BodyAdjuster form says:

This field may support tokens or Twig template syntax – please check the supplied default policy for possible values.

We should instead help people out. This issue covers the tokens - see #3270408: Email body policy form should show available token browser and TWIG variables for the tokens.

Steps to reproduce

Edit a policy and add a body element

Workaround

1. Look at the default installed policy - this will often use most of the available variables
2. Look at the code in the EmailBuilder. The variables are set with setVariable().

Proposed resolution

1) Add new parameter to the @EmailBuilder annotation: variables = array variable name => description.
2) Make the EmailBuilder definition available in the form

• Add MailerPolicy::getBuilderDefinition()
• Deprecate MailerPolicy::getCommonAdjusters(), as it's easy to get from the above
• In PolicyEditForm::form(), call $this->entity->getBuilderDefinition() and store the result in the form.

3) Use the annotations in BodyEmailAdjuster::settingsForm(). Display the variables with '#type' => 'details', and set #value based on the variables annotation.

Example

ContactPageEmailBuilder:

variables = {
  "form" = "BLAH",
  "form_url" = "BLAH",
  "recipient_name" = "BLAH",
  "recipient_edit_url" = "BLAH",
}

Remaining tasks

User interface changes

API changes

Data model changes