YubiKey: Clean-up master seed challenge

* Tweak the logic so it more closely resembles other code (i.e.
  trasnformKey()). Matches existing style better.
* Save the challengeResponseKey in the database structure so that
  it can be referred to later (i.e. database unlocking).

Signed-off-by: Kyle Manna <kyle@kylemanna.com>
YubiKey: Fix database locking

* Save the master seed upon first challenge so it can be used as a
  challenge at a later point.
* When verifyKey() is called, verify that the challenge is successful.
* Uncheck YubiKey box to not leak information about how the database is
  protected.

Signed-off-by: Kyle Manna <kyle@kylemanna.com>