Make headline images explicit, add asset credits

- related_assets gains a `headline` boolean (DB-enforced: at most one
  per page), replacing "first image by position" as the headline
  rule. A rake task backfills the current first image on every live
  head/draft, so nothing changes visually until an editor changes it.
- The image picker sidebar gets a star toggle reflecting the flag;
  the TinyMCE inline-image picker's badge now reads it too, instead
  of assuming position 0.
- No headline chosen (or none attached) now falls back to the
  gallery-count caption itself becoming the lightbox trigger, instead
  of the gallery being unreachable.
- Assets gain creator, source_url, and license_key (against a new
  config/asset_licenses.yml dictionary). asset_credit renders a
  degrading attribution line, reused as a hidden per-image glightbox
  caption so credit is one click away for every image, not only the
  headline's always-visible one.
- Fixed: asset thumbnails rendered unconditionally regardless of
  whether a real variant exists on disk. Asset#has_variant? checks
  file existence, not content type -- some legacy PDFs have real
  pre-rewrite thumbnails a content-type check would have hidden.
- assets#new/edit rebuilt onto the same node_description/node_content
  layout as assets#show, picking up the three new fields in the
  process.

1 files changed, 8 insertions, 0 deletions

diff --git a/app/models/concerns/file_attachment.rb b/app/models/concerns/file_attachment.rb
index 3c094561..e7a33c55 100644
--- a/app/models/concerns/file_attachment.rb
+++ b/app/models/concerns/file_attachment.rb
@@ -52,6 +52,14 @@ module FileAttachment
     build_upload_proxy
   end
 
+  def has_variant?(style)
+    upload_file_name.present? && File.exist?(file_path(style))
+  end
+
+  def previewable?
+    has_variant?(:medium)
+  end
+
   private
 
   def build_upload_proxy