Add option to disable creating comment migrate plugin

Problem/Motivation

Unlike the other migrations comments are just imported automatically. Add a stage to the import wizard UI.

There is already a /src/Form/CommentSelectForm.php but it is not integrated!

This could also help as may avoid having glitchy xml comments be parsed if they are to be dropped anyway.

There is some bug in this area anyway (incorrect migration plugin set up) so switching it off is good. see 🐛 Undefined array key "migration" in findMigrationDependencies() Active

Proposed resolution

Integrate this Form page (to come after the pages / posts) in the wizard.

Remaining tasks

- update ImportWizard.php getOperations
- test CommentSelectForm.php
- change WordPressMigrationGenerator.php to add a condition around L245 like
if ($this->configuration['image_field']) {

User interface changes

new wizard page in import process

API changes

migration plugin entity for comments may go away

Data model changes

additional key on var array this->configuration inside WordPressMigrationGenerator