Cannot create links yet

[tcr]

This should be simple to support, because Bold and Italic are already partially supported on the client. The difference here:

1. The client needs to detect that the Link button was pressed, and intercede by showing a modal dialog asking for a URL.
2. That modal dialog needs to be designed and implemented.
3. Then the result needs to be sent to the server (the button pressed by client command can possibly be extended with a string "data" argument) and used as the value for the Style::Link style.