Feature/xw 111/koophulpje

[lencodes]

This pull request adds theme switcher logic.

• Theme switcher top-bar component, which only shows when the theme switcher is activated in the config;
• IF the theme switcher is enabled via domain name OR .env variables, it's forced ON, even if the JSON-config says it's off;
• The theme, OIDN, Gateway-URL, Footer content and more is all updated when the switcher is used.

To test:

• Disable GATSBY_ENV_VARS_SET in the .env.development (or keep enabled and enable GATSBY_SHOW_THEME_SWITCHER as well);
• Uncomment // case "localhost.nl": // development purposes in useEnvironment() hook;
• Uncomment // case "localhost": // development purposes in getConfig().

[remko48]

Nice