Updating permissions checks in the AJAX handler.

[ideadude]

Description

Added additional permissions checks to callback functions triggered via AJAX where appropriate.

We also removed some deprecated methods of the LLMS_AJAX_Handler class, including:

• query_students
• create_section
• get_course_sections
• get_course_section
• update_course_section
• create_lesson
• get_lesson_options_for_select
• add_lesson_to_course
• get_course_lesson
• update_course_lesson
• remove_course_lesson
• delete_course_section
• update_section_order
• update_lesson_order

We also added the function llms_current_user_can_edit_product() which takes a product_id (for a course or membership) and returns if the current user can edit that course or membership. This makes the permissions checks for products more clear.

How has this been tested?

We've created AJAX calls for each method and confirmed the new or existing permissions checks keep unwanted actions from occuring.

We also tested across the whole application to make sure these changes aren't affecting legitimate use of the software.

Checklist:

• This PR requires and contains at least one changelog file.
• My code has been tested.
• My code passes all existing automated tests.
• My code follows the LifterLMS Coding & Documentation Standards.