Add workflow to create an experiment workspace

Part of #542 (closed). See: https://salsa.debian.org/freexian-team/debusine/-/blob/devel/docs/reference/devel-blueprints/experiment-workspaces.rst

• create a server task that: (!1618 (merged))
  • creates a new workspace, with an expiration_delay
  • sets the new workspace as inheriting an existing workspace
  • creates a new group containing a given user
  • sets the new group as OWNER of the new workspace
  • copies a list of WorkflowTemplates from an existing workspace to the new workspace
• create a workflow that runs the server task (!1632 (merged))
• test that one cannot use a signing key from a WorkflowTemplate copied from the source workspace (!1633 (merged))