A library save carries a rejected set's media metadata live #2664

Part of #2427 (F-MED). Found by the review of #2549.

The admin library's asset view (MediaAssetView.razor) fills its form from the latest version and saves on top of it. When the latest version is one a change set proposes, pending or rejected, a person who changes only the tags saves a new version that still carries the set's alt text, caption and file name. Library writes take effect at once (#2549), so metadata a reviewer rejected goes live without anyone deciding it should.

Steps:

1. A published image has no alt text.
2. An agent's set proposes version 2 with alt text, and a person rejects the set.
3. A person opens the image in the library and changes only its tags.
4. The save stores version 3 with the rejected alt text, and the alt text check passes on every page that uses the image.

Scope

• Base the library form on the live version (IMediaLibrary.LiveVersionAsync), and say on the page when a change set proposes a newer version.
• Or refuse a library save over a version that only a set proposes, with a message that points at the set.

Out of scope

• Editing a set's proposed version from the library.

Acceptance

• Saving an image's details in the library after a set proposing other metadata was rejected keeps the live metadata, apart from the fields the person changed.
• The page shows when a pending set proposes a newer version.
• Tested in the admin view tests.