v2019.4: Release notes

Rendered here: https://em.pages.apertis.org/apertis-website/

Links are broken, so here are the pages involved:

• https://em.pages.apertis.org/apertis-website/release/v2019.4/releasenotes/
• https://em.pages.apertis.org/apertis-website/release/v2019.5/release_schedule/
• https://em.pages.apertis.org/apertis-website/policies/releases/