Factorisation et correction des dropdowns (#511)

assets/sass/_theme/_variables.sass

assets/sass/_theme/design-system/button.sass

assets/sass/_theme/design-system/dropdown.sass

assets/sass/_theme/design-system/footer.sass

assets/sass/_theme/design-system/nav.sass

assets/sass/_theme/hugo-osuny.sass

assets/sass/_theme/sections/diplomas.sass

assets/sass/_theme/utils/normalize.sass

assets/sass/_theme/utils/shame.sass

layouts/partials/diplomas/diplomas-select.html

layouts/partials/footer/i18n.html

layouts/partials/header/i18n.html