Accessibility: Adds an alternative, high-contrast color-scheme, closes #96

app/controllers/settings/accessibility.php

app/views/resources/print/clipboard_rooms.php

app/views/settings/accessibility.php

app/views/settings/general.php

db/migrations/5.2.14_add_high_contrast_config_entry.php

lib/navigation/AvatarNavigation.php

lib/navigation/ProfileNavigation.php

lib/phplib/Seminar_Auth.class.php

lib/seminar_open.php

public/assets/images/icons/blue/accessibility.svg

public/index.php

public/logout.php

resources/assets/stylesheets/highcontrast.scss

resources/assets/stylesheets/less/index.less

resources/assets/stylesheets/mixins/colors.less

templates/index_nobody.php

webpack.common.js