feat(updates): add featured image and gallery to Updates

[danieltruong]

Staff can add images to EPIC Updates, and the publish picker is fixed.

Update editor:

• A featured image, and a gallery of up to 5 photos. Each can be uploaded new or picked from the project's images. Photos can be reordered and removed.
• Every image needs alt text. Caption is limited to 300 characters and credit to 150, with counters. A message says when alt text is missing.
• Uploads accept JPEG, PNG, WebP and GIF up to 10 MB, and check that the file extension matches its type. Focus moves to the new row's alt text after an upload, unless the user has moved elsewhere.
• Publish is blocked while an image or attachment is not public, with the reason shown.
• The picker offers only public images from the same project. It uses radio buttons when one image can be picked.

Other:

• file-upload takes an accept list and shows a hint. Its browse link is now a button (project document upload, notification upload, add comment).
• The project Updates list sorts by date added by default, and keeps sort and page size when the page changes.

Needs the eagle-api Update image support (bcgov/eagle-api#907).

Tests: new specs for the image field, the picker, the image helpers, the update rules, file-upload and the project Updates list. The editor spec gains DocumentService and LoggingService spies.