add scroll progress bars & fallback images

district/migrations/0253_districthomepage_fallback_image.py

...grations/0254_districtarticlepage_show_scroll_progress.py

district/models.py

district/templates/district/district_person_page.html

...tions/migrations/0048_electionshomepage_fallback_image.py

...rations/0049_electionsarticlepage_show_scroll_progress.py

elections/templates/elections/elections_article_page.html

main/migrations/0107_mainhomepage_fallback_image.py

main/migrations/0108_mainarticlepage_show_scroll_progress.py

shared/models/main.py

shared/static/styleguide2/main.css

shared/static/styleguide2/main.js

shared/templates/styleguide2/article_page.html

shared/templates/styleguide2/contact_page.html

...ncludes/molecules/candidates/candidate_secondary_box.html

...yleguide2/includes/molecules/contact/center_contacts.html

.../styleguide2/includes/molecules/contact/person_badge.html

.../styleguide2/includes/molecules/popouts/popout_point.html

...tyleguide2/includes/organisms/cards/people_card_list.html

...s/styleguide2/includes/organisms/header/photo_header.html