Migration to MUI v5

See v2 Architecture for an explanation of the n and n-legacy branches

The upgrade to MUI v5 should ideally be done on the n branch - in combination with code cleanup and refactoring.

The n branch is not yet ready for general development (but will be latest on the 6th of August 2022).

In the meantime, features can already be upgraded to MUI v5 on the n-legacy branch (though I would hold off on doing any other code refactoring until the features are moved to the n branch).

CSS modules

Migrating to MUI v5 also includes replacing the CSS module styles (*.module.css files) with the MUI CSS-in-JS styling system.

It also includes replacing particularly badly implemented styles with more sensible CSS rules.