misc/coverage: Move feature into its own django application

swh/web/archive_coverage/__init__.py

swh/web/archive_coverage/templates/archive-coverage.html

swh/web/archive_coverage/urls.py

swh/web/archive_coverage/views.py

swh/web/config.py

swh/web/misc/urls.py

swh/web/templates/homepage.html

swh/web/tests/archive_coverage/__init__.py

swh/web/tests/archive_coverage/test_app.py

swh/web/tests/archive_coverage/test_coverage.py