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, 1 insertions, 1 deletions

diff --git a/app/views/assets/index.html.erb b/app/views/assets/index.html.erb
index 8c355617..ff1300a2 100644
--- a/app/views/assets/index.html.erb
+++ b/app/views/assets/index.html.erb
@@ -17,7 +17,7 @@
   </tr>
 <% @assets.each do |asset| %>
   <tr>
-    <td><%= image_tag asset.upload.url(:thumb), style: "max-width: 100px; max-height: 100px;" %></td>
+    <td><% if asset.has_variant?(:thumb) %><%= image_tag asset.upload.url(:thumb), style: "max-width: 100px; max-height: 100px;" %><% end %></td>
     <td><%= link_to asset.name, asset.upload.url %></td>
     <td><%= asset.upload.content_type %></td>
     <td><%= link_to 'Show', asset %></td>