Move server resourcepack confirmation screen to dialog system

It would be cool if the server resourcepack confirmation screen also used the new dialog system, to allow servers to display some additional information or actions there.

For example, a server could provide a longer, formatted, explanation on why a resourcepack is required, a "Join without resourcepack" button with a disclaimer that things might look broken, or a "Attempt download again" screen. 

Currently it is only possible to provide a simple message there, and with dialogs being available in the config phase already this would be a nice addition.