If built-in proxy is used, inform user

[csarven]

If a user doesn't have a solid:preferredProxy in their profile, built-in DO.C.ProxyURL value will be used to make requests when proxy is required (see also https://github.com/linkeddata/dokieli/issues/230 ). If/when built-in proxy needs to be used, show a notice to the user indicating their choices eg. continue to use proxy at this URL, or cancel request. Offer an option to remember the user's choice (until they sign out).