Yubikey: Factor out combobox code

* Move the addItem() call to populate the yubikey comboboxes to the
  Yubikey library.
gui: yubikey: Use Yubikey's add combo box items

* Use the function in the library.
Yubikey: Use 64 bytes for challenge test

* Some Yubikeys may be configured for 64 byte challenge responses, so
  send 64 bytes to work with these devices.