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, 14 insertions, 0 deletions

diff --git a/config/locales/en.yml b/config/locales/en.yml
index 8a8acc15..1afcf219 100644
--- a/config/locales/en.yml
+++ b/config/locales/en.yml
@@ -68,3 +68,17 @@ en:
     revision_created: "created %{date} by %{actor}"
     revision_published: "published %{date} by %{actor}"
     revision_restored: "restored %{date} by %{actor}"
+
+  open_gallery: "Open gallery"
+  asset_licenses:
+    cc0: "CC0"
+    cc_by_4: "CC BY 4.0"
+    cc_by_sa_4: "CC BY-SA 4.0"
+    public_domain: "Public domain"
+    used_with_permission: "Used with permission"
+    own_work: "Own work"
+
+  asset_credits:
+    photo: "Photo %{name}"
+    by: "by %{creator}"
+    licensed_under: "Licensed under %{license}"