Added release date and Newsletter plugin

final 1.4

Update CHANGELOG.md

Named FP1.4: "Notturno"

Preparing the 1.3.1 release: Version bump, update changelog

Final release of 1.3 \o/

Proud version bump - we have our first RC of FlatPress 1.3! \o/

Fixes #297

Fixes: Undefined array key "xyz"
Fixes: Attempt to read property "value" on null
Fixes: Trying to access array offset on value of type null

bugfix release

Final version 1.2 "Legato"