branding, manual, and changelog updates for 2.0

fix #838

fix #836

fix #833

fix #831

fix #829

also, handle nickname targets in znc.in/playback

change "last signoff" tracking to "last seen"

Explicit quit and ping timeout behave the same way,
but reattach after abandoning/losing the previous session
(without the break being detected server-side) is more aggressive
about replaying missed messages, at the cost of potential duplication.

Make docker build with go 1.14

partial fix for #812

fix #821, maybe

Or try to, anyway

fix #781

fix #814

tweaks to persistent history

fix handling of history grace periods

reduce recommended cloak bits from 80 to 64

From testing on darwin; shaves 3 characters off the cloaked hostname,
and 64 bits of entropy is fine (a collision is expected after 2**32
~= 4 billion distinct CIDRs).

raise passphrase length limit to 300 bytes

For irctest compatibility.

Fixes #775