Change default secretLength length to 26

[mokraemer]

RFC and freeOTP require at least 128 bit, better 160bit and more.

newer freeOTP versions show warnings if the length is less than 128 bit.

[paramazo]

Call createSecret(26) works, but the verifyCode function always fails with this generated 128bit secret.

// Only multiple of 8 will work, i.e. 24 or 32. Use 32 if apps like freeotp should not print a warning message