course categories: Fix many bugs with category editing and permissions. Clean up...

course categories: Fix many bugs with category editing and permissions. Clean up code.

Bugs: MDL-17479, MDL-16426, MDL-16063, MDL-16013, MDL-15658, MDL-15556, MDL-15161, MDL-14925, MDL-13742, MDL-11557.

* Simplify category editing permissions to just moodle/category:manage and moodle/category:seehiddencategories.
* Enforce those correctly. (Note MDL 17502 is still outstanding.)
* Don't screw up category sort order when you just edit name or description.
* Niceties like where redirects go when you cancel or submit forms.
* Make sure a global course creator can see the site admin block.
* Don't allow a category to be made the child of one of its children!
* General code cleanup to bring key files more in line with best pracitice.

Apologies for the fact it is one big patch, rather than a series of smaller patches. However, categoryedit.php, category.php and index.php where in pretty bad shape and needed significant cleaning up. categoryedit.php, in particular, was almost completely rewritten.

Merged from MOODLE_19_STABLE.