[ADD] gallery blok to ArticleMixin

...rict/migrations/0051_alter_districtarticlepage_content.py

district/templates/district/base.html

region/migrations/0026_alter_regionarticlepage_content.py

region/templates/region/base.html

shared/blocks.py

shared/models.py

shared/templates/styleguide/2.3.x/blocks/gallery_block.html

uniweb/migrations/0023_alter_uniwebarticlepage_content.py