Add an image gallery shortcode

layouts/_shortcodes/gallery.html

@@ -0,0 +1,20 @@
+{{- /*
+Image gallery: square thumbnails, two per row on wide screens, that open the full image in a
+lightbox (assets/js/gallery.js) with the alt text and credit. Without JavaScript the
+thumbnails link to the full image.
+
+  {{< gallery >}}
+  {{< gallery-image src="photo1.jpg" alt="What the photo shows" credit="© Photographer" >}}
+  {{< gallery-image src="photo2.jpg" alt="…" credit="…" >}}
+  {{< /gallery >}}
+*/ -}}
+<div class="gallery" data-gallery
+  data-label-dialog="{{ i18n "gallery_dialog" }}"
+  data-label-close="{{ i18n "gallery_close" }}"
+  data-label-previous="{{ i18n "gallery_previous" }}"
+  data-label-next="{{ i18n "gallery_next" }}">
+<ul class="gallery__list">
+{{- .Inner }}
+</ul>
+</div>
+{{- /* chomp */ -}}