Optimize and unify PHP, part 1

See merge request !12

Change repo url in theme

See merge request !20

Adjust URLs in README to renamed repo

See merge request !19

Resolve "Set title of 404 page to something general"

Closes #14

See merge request fachschaft/wp-theme!18

Cherry pick unified things

See merge request fachschaft/wp-theme!16

It will always be overriden by code in our template

This has the added extra benefit that it also takes care of handling passwort protected pages

See https://developer.wordpress.org/reference/functions/the_date/

Clean functions.php

See merge request fachschaft/wp-theme!15