US-19 "Save as draft" functionality

User story

As a librarian, I want the option to save an exhibition as a draft so that I can save my progress and continue later.

Should be implemented alongside US-11

Prerequisites

• The editor dashboard exists.
• Exhibitions can be created.

Acceptance criteria

• The exhibition editor has the option to save it as a draft.
• Saving is done correctly and the exhibition can be reopened and edited.

Sprint Ready Checklist

1. Acceptance criteria defined
2. Team understands acceptance criteria
3. Team has defined solution / steps to satisfy acceptance criteria
4. Acceptance criteria is verifiable / testable
5. External / 3rd Party dependencies identified