Add YouTube thumbnail support

Fixes #359

app/scss/gallery.scss

@@ -52,10 +52,21 @@
 	justify-content: center !important;
 	cursor: pointer;
 
+	img {
+		position: absolute;
+		top: 0;
+		right: 0;
+		bottom: 0;
+		left: 0;
+		object-fit: cover;
+		z-index: 10;
+	}
+
 	.fa-play {
 		display: block;
 		font-size: 200%;
 		color: #f44;
+		z-index: 20;
 	}
 
 	&:hover {
@@ -72,6 +83,7 @@
 		right: 0.5rem;
 		color: #555;
 		font-size: 80%;
+		z-index: 30;
 	}
 }