Portal-level default assignee / curator for idea portals

Add the ability to assign a single "portal owner" or catch‑all default assignee at the idea portal level (not just per workspace). This person would be responsible for overall portal curation — managing how the portal is presented, overseeing users and comments, and handling unassigned or "needs review" ideas — even when multiple people are assigned ideas that come through that portal.

Related need: Make this portal owner/assignee available as a field that can be included in reports so teams can easily see ownership and identify gaps across portals and workspaces.