Disallow selecting non-URI entry in guided-choice textfield

[tkuhn]

Typing some text in a guided-choice textfield not only filters the responses from the API but also shows that text itself as a possible choice itself. That should only be the case if the entered text has the form of a (http/https) URI.

I am not sure where this entry of the currently typed text comes from. It doesn't seem to be there in plain select2 textfields when used outside of Wicket, and I don't see any place where it's added in the our own code...