[ADD] allow district, region to customize menu in HP

district/migrations/0040_districthomepage_menu.py

district/models.py

district/templates/district/base.html

district/wagtail_hooks.py

region/migrations/0015_regionhomepage_menu.py

region/models.py

region/templates/region/base.html

shared/blocks.py

shared/models.py

shared/templates/styleguide/2.3.x/menu_item.html

shared/templates/styleguide/2.3.x/menu_parent.html