Hide secret URL from post

Taken from https://github.com/burntout/d-note/commit/e09e4a8147097a99cd273945abdb035e67120f84
Change 16-byte filename, AES-256 and HMAC-SHA512

This is a massive change to maximize security and bring the URL back to 22
base64 characters. This push would not be possible without Alan Dawson -
https://github.com/burntout/.
fix some install steps