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, 9 insertions, 0 deletions

diff --git a/app/models/related_asset.rb b/app/models/related_asset.rb
index 8f164600..62000cc8 100644
--- a/app/models/related_asset.rb
+++ b/app/models/related_asset.rb
@@ -5,4 +5,13 @@ class RelatedAsset < ApplicationRecord
   acts_as_list :scope => :page_id
 
   default_scope -> { order("position ASC") }
+
+  validate :headline_only_for_images
+
+  private
+
+    def headline_only_for_images
+      return unless asset
+      errors.add(:headline, "can only be set on image assets") if headline? && !asset.image?
+    end
 end