Protect tables from course cascade deletion.

[nhoobin]

When cleaner_courses is enabled during the cascade deletion some important tables have rows removed from them which can impact using the site.

One example would be removing rows from the table {my_pages} which would result in not being able to view the page at $CFG->wwwroot/my

[brendanheywood]

Would be good to actually get a complete list of where the cascade goes so we have a full list of things to check