Pad secret token correctly

After converting base 16 (hex) string of length 16 to integer and then to
base 36 we can end up with a shorter string due to implicit zeroes on
leading positions in the intermediate integer. This ensures
"00000000000000000000000000000000", "ffffffffffffffffffffffffffffffff"
and everything in between result in a string of length 25 after converting to
base 36.