Create a management command to rename a scope

See docs/reference/devel-blueprints/scoped-urls.rst:

Implement a management command to rename a scope.

This is likely to be a simple management command that changes Scope.name, since only foreign keys to scope are used to refer to scope at the database level.

This is needed to complete the migration path from unscoped to scoped debusine: database migrations will create a debusine scope and assign everything to it, and admins will then need a way to rename the debusine scope to what they need (for example: debian in case of debusine.debian.net)