migrate uniweb blocks with new counterparts

shared/blocks/base.py

...rations/0071_alter_uniwebflexiblepage_content_and_more.py

uniweb/migrations/0071_auto_20240626_1235.py

uniweb/migrations/0072_auto_20240626_1235.py

...rations/0073_alter_uniwebflexiblepage_content_and_more.py

uniweb/models.py