misc/banners: Move feature into its own django application

assets/src/bundles/webapp/webapp.css

swh/web/banners/__init__.py

swh/web/banners/templates/fundraising-banner.html

swh/web/banners/templates/hiring-banner-iframe.html

swh/web/banners/templates/hiring-banner.html

swh/web/banners/urls.py

swh/web/banners/views.py

swh/web/config.py

swh/web/misc/urls.py

swh/web/templates/layout.html

swh/web/tests/banners/__init__.py

swh/web/tests/banners/test_app.py

swh/web/tests/banners/test_fundraising.py