IIIF: set startCanvas from default_preview

[dfdan]

❤️ Thank you for your contribution!

Description

If a default preview file has been selected, this will be added as the manifest's startCanvas.

Checklist

Ticks in all boxes and 🟢 on all GitHub actions status checks are required to merge:

• I'm aware of the code of conduct.
• I've created logical separate commits and followed the commit message format.
• I've added relevant test cases.
• I've added relevant documentation.
• I've marked translation strings (for relevant code).
• I've followed the CSS/JS and React guidelines (for relevant code).
• I've followed the web accessibility guidelines (for relevant code).
• I've followed the user interface guidelines (for relevant code).
• I've identified the copyright holder(s) and updated copyright headers for touched files (>15 lines contributions).
• I've NOT included third-party code (copy/pasted source code or new dependencies).

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[github-actions]

This PR was automatically marked as stale.

[dfdan]

rebased against master

[dfdan]

Rebased. IIIF tests passing, so this should be mergable once other issues causing test failures are resolved.

[github-actions]

This PR was automatically marked as stale.