Add footer links to uniweb + districts

district/migrations/0109_districthomepage_footer_links.py

district/models.py

district/templates/district/base.html

senate/migrations/0006_senatehomepage_footer_links.py

senate/models.py

senate/templates/senate/base.html

shared/blocks.py

shared/models.py

shared/templates/shared/blocks/footer_links_block.html

uniweb/migrations/0042_uniwebhomepage_footer_links.py

uniweb/models.py

uniweb/templates/uniweb/base.html