Rewrite theme for MDL4.x

Complete rewrite of the theme for Moodle 4

New theme settings:

• navbar color
• additional login links
• footer columns are now more flexible, easier to edit and translate

On the code side:

• split SCSS into more manageable files
• rewrite of the course categories colorisation to remove the hardcoded id and allow more flexibility
• made use of templates where possible

Other stuff:

• accessibility setting in the user menu
• dark mode (wip)