add classes to blocks, add custom css

district/blocks.py

district/migrations/0239_districthomepage_custom_css.py

district/migrations/0240_alter_districthomepage_content.py

district/models.py

district/templates/district/district_calendar_page.html

...plates/district/district_geo_feature_collection_page.html

district/templates/district/district_pdf_page.html

shared/templates/styleguide2/base.html

...plates/styleguide2/includes/molecules/menus/carousel.html

.../styleguide2/includes/organisms/header/simple_header.html

...mplates/styleguide2/includes/organisms/layout/navbar.html

shared/templates/styleguide2/simple_page.html