More work on URL logins, and add wallet export support

URL logins should now work from the client side.

Also, the File->Export Wallet menu item is now fully functional.
URL Login enhancements

Various improvements to the URL login flow
* Do not allow users to log in to sites that have not registered on the
blockchain.
* Make the login authorization dialog when only one account is created a
sheet on OSX.
* Add the client's username to the login request.
Do nothing with privileged URLs when no wallet is open