Svg icon api sass function and upgrade of all styles

apps/accessibility/css/themedark.scss

apps/encryption/css/settings-personal.scss

apps/files/css/files.scss

apps/files_external/css/settings.scss

core/Controller/SvgController.php

core/css/functions.scss

core/css/header.scss

core/css/icons.scss

core/css/styles.scss

core/css/variables.scss

core/img/actions/audio-off-white.svg

core/img/actions/audio-white.svg

core/img/actions/caret-dark.svg

core/img/actions/caret.svg

core/img/actions/checkmark-color.svg

core/img/actions/checkmark-white.svg

core/img/actions/close-white.svg

core/img/actions/confirm-white.svg

core/img/actions/delete-hover.svg

core/img/actions/delete-white.svg