Refactor the `footer` Sass partial
Refactor the `footer` Sass partial

- Replace the grid-based layout by a flexbox-based one.
- Ensure that, if flexbox is not supported, people get a functional centered one-column layout.
- Add comments
- Remove duplicate code
- Remove useless code
- Cleaning and formatting

monnaie-valheureux/radisse,monnaie-valheureux/radisse