Encode scope in debusine URLs

References: #395 (closed), #486 (closed), permission design blueprint.

In design discussions we decided to encode the scope in Debusine URLs.

URL parsing and generation needs to be adapted to encode the current scope. We can assume the "debusine" fallback scope as a default until we have actual tracking of the current scope (#488 (closed))

(report time in #395 (closed))