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/public/stylesheets/ccc.css b/public/stylesheets/ccc.css
index a07cc4d9..8358d06c 100644
--- a/public/stylesheets/ccc.css
+++ b/public/stylesheets/ccc.css
@@ -747,3 +747,11 @@ div.teaser_ruler {
   float: right;
   margin-left: 1rem;
 }
+
+#asset_credits .glightbox-desc {
+  display: none;
+}
+
+#asset_credits .headline_credit {
+  display: block;
+}