Add a high contrast preference in user preferences

For some low vision users, the 4.5 contrast we make sure to fulfill on the required elements in the Iceberg color theme is not enough for comfort. It would be an improvement to propose a high contrast alternative to the current color theme.

As far as I know, we don't have a high-contrast color theme yet, so we'd need to create it, and choosing which high contrast theme to use from administration would make little sense. In my opinion, it should just be a toggle in the user preferences, which would allow the user to always use the same theme.

The main point of this theme would be to ensure WCAG AAA contrasts requirements, or even better, even if some of the softness of the UI is lost

A further development of this idea would be to adapt the color themes depending on the `prefers-contrast` media query (https://developer.mozilla.org/en-US/docs/Web/CSS/@media/prefers-contrast).