main: articles refactor

district/wagtail_hooks.py

...12_alter_mainpeoplepage_people_delete_mainarticlespage.py

main/migrations/0013_rename_mainworkpage_mainarticlespage.py

...igrations/0014_alter_mainarticlespage_options_and_more.py

main/models.py

main/templates/main/includes/work_article_preview.html

main/templates/main/main_program_page.html