Make the dialog buttons for URL and FilePicker the same

Currently the Submit button is disabled for the file picker, but not for the URL picker. Instead, the URL picker has a client-side popup message explaining that it won't work until you pick a URL.

We should adopt this behavior for both dialogs. The file picker is a bit confusing because the disabled styling looks like the enabled cancel button. So its unclear it cannot be clicked and then if you click it it, it's also unclear why it does not work. Additionally, making these 2 dialogs behave the same improves consistency.