Fine tune breadcrumbs (fixes #1190)

Visible at https://playground-2470.debusine.dev/

• Example work request: https://playground-2470.debusine.dev/debusine/Playground/work-request/10/
• Example workflow: https://playground-2470.debusine.dev/debusine/Playground/work-request/11/
• Example collection: https://playground-2470.debusine.dev/debusine/Playground/collection/debian:environments/debian/
• Example collection search: https://playground-2470.debusine.dev/debusine/Playground/collection/debian:environments/debian/search/
• Example group page (requires login): https://playground-2470.debusine.dev/debusine/-/groups/debusine-Playground-owner/

I have not touched the "Create artifact and create work request pages", as I agree we can get rid of them (filed as #1192)