classes/Options_V2: Update the permalink when changing the course slug

[bernhardkaindl]

This feature is already released
But it was initially not working for existing courses, See bug #576.

This code is originally from @shewa12's PR about it.

Renaming the course base slug of existing courses is missing. They stay with their old slug!!!

These changes (of this Pull Request) which @shewa12 made, may do that, right?

These changes have not been merged yet!!!

	array(
		'key'         => 'course_permalink_base',
		'type'        => 'text',
		'label'       => __( 'Course Slug', 'tutor' ),
		'placeholder' => __( 'Course Slug', 'tutor' ),
		'label_title' => '',
		'default'     => tutor()->course_post_type,
		'desc'        => __( 'Update the permalink when changing the course slug', 'tutor' ),
	),

The same renaming would also have to be done when the lesson base slug changes, which is also not done.