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

[bernhardkaindl]

Also, fold a variable that was only used once.

This is @shewa12's #549, rebased (with conflicts resolved) to the current dev branch.

[shewa12]

This feature is already released

[bernhardkaindl]

This feature is already released

@sazedul-haque, or someone else, please have a look at this:

Yes, but not working for existing courses!!!!!!!!!!!!!!!! See bug #576!!!

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.

[bernhardkaindl]

@shewa12 I guess handling changes to the course and lesson base slugs may have been fixed meanwhile, but it would be great if you could communicate about it.