Recognise the renamed S3 Media plugin for remote PDFs.

The 16.8.0 remote-PDF integration keyed off S3VIDEO_TABLE and
s3video_fetch_to_file(), but that plugin became S3 Media in its 16.3.0
release. On upgraded galleries pdfcp_remote_pdf_url() returned null, so
the "PDF cover page" tab never appeared for remote PDFs and the feature
was silently unavailable.

Prefer the S3 Media names, falling back to the S3 Video ones so galleries
that have not upgraded keep working.