Move auth constants to nexus.conf

The following constants from sever/router/auth.go should be moved to the global nexus.conf file:

    // Access token expiration time.
    accessTokenExpirationTimeInHours = 12

    // Number of login attemps after which an account is locked.
    maxloginAttempts = 5

    // How long an account is locked for.
    lockedTimeInHours = 4.0