Test scope support more extensively and possibly get rid of DEBUSINE_DEFAULT_SCOPE

When we have implemented scope support, we relied heavily on having a default scope named "debusine". But we didn't test many things with a default scope having another name or with multiple scopes. This ticket is there to improve this and do a review of what's left to do.

• Rename the default scope without renaming DEBUSINE_DEFAULT_SCOPE and see whether can still run everything in that scope (in particular debusine-admin commands)
• Get rid of DEBUSINE_DEFAULT_SCOPE entirely (migration should use scope_id=1 instead)
• Add a second scope and see whether we can be added to that scope and work in that scope