Add lightbox media queries.

Add media queries to hide the image actions if on mobile (probably
unnecessary to have), and make the image description (name, author)
close to full width.

static/styles/overlay.css

@@ -145,3 +145,13 @@
 #overlay .clear-float {
     clear: both;
 }
+
+@media (max-width: 450px) {
+    #overlay .image-actions {
+        display: none;
+    }
+
+    #overlay .image-description {
+        width: calc(100% - 100px);
+    }
+}