Make external-link icon next to "guide" lighter

See merge request !55

This prevents the messages.js script to cause errors on other pages,
because it can't find the div.

flush stdout after print

See merge request !53

Also use HTML attributes to control all this.

* Change name from 'hashlib' to 'pbkdf2'
* Use random bytes as salt (not hexlified)
* Write number of iterations in hash string