refactor Admin UI

This introduces a new dokuwiki\Ui namespace and refactors the Admin
screen into a Ui class. The ultimate goal is to split up the big,
complex functions in inc\html.php in better maintainable classes in the
Ui namespace. This is the first go at it. Others function->class
conversions should follow.

This also switches the icons for our base admin plugins to inline SVG.
(files and styling not included, yet).