Cleanup include tags

_includes/course.html

_includes/date.html

_includes/header.html

_includes/menu_single.html

_includes/menu_sub.html

_includes/pagination.html

_includes/people_chip.html

_includes/people_list.html

_includes/post_link.html

_includes/post_meta.html

_includes/psitem.html

_includes/semester.html

_layouts/course.html

_layouts/person.html

_layouts/post.html

_layouts/project.html

_layouts/system.html

_posts/2015-05-08-OpenDreamKit.md

_posts/2015-07-22-New-Demo-OEIS.md

_posts/2016-10-01-KWARC-established.md