The Flex component should be correspond with designsystem sizes and breakpoints

[paal2707]

Description

Per now the Flex component is a duplicate of the MUI Grid. We need to decide which sizes, spacing and breaking points we should use and follow the "Designsystemet" to a greater extent. This will cause refactoring of the styling in components in several places in the application.

There are multiple places breakpoints have been defined, and they don't all match each other:

• altinnAppTheme.tsx
• useDeviceWidths.ts
• (the new, at the time of writing this issue) CSS for the Flex component

All of these should end up using the same values.