Fixes for #269:
* Only one admin table for all announcements
* Field `author` is no longer mandatory
See comment https://github.com/softwerkskammer/NeuePlattform-Implementierung/pull/278#discussion_r5245112
Don't render a link if author is 'automatisch'
Merge pull request #280 from fdeberle/announcements

Announcements: Verwaltung-Nachrichten zwei Tabellen geht nicht (#269)