Add optional black below-footer content

.../migrations/0110_districthomepage_footer_extra_content.py

district/models.py

district/templates/district/base.html

...ations/0011_senatcampaignhomepage_footer_extra_content.py

senat_campaign/models.py

senat_campaign/templates/senat_campaign/base.html

...eb/migrations/0042_uniwebhomepage_footer_extra_content.py

uniweb/models.py

uniweb/templates/uniweb/base.html