client/general: show empty thumbnails as grid

client/css/main.styl

@@ -177,6 +177,10 @@ a .access-key
         height: 100%
     width: 20px
     height: 20px
+    &.empty
+        background-image: url('/img/transparency_grid.png')
+        background-repeat: repeat
+        background-size: initial
 
 .flexbox-dummy
     height: 0 !important

client/js/util/views.js

@@ -42,10 +42,12 @@ function makeRelativeTime(time) {
 function makeThumbnail(url) {
     return makeNonVoidElement(
         'span',
-        {
-            class: 'thumbnail',
-            style: `background-image: url(\'${url}\')`,
-        },
+        url ?
+            {
+                class: 'thumbnail',
+                style: `background-image: url(\'${url}\')`,
+            } :
+            {class: 'thumbnail empty'},
         makeVoidElement('img', {alt: 'thumbnail', src: url}));
 }