comments: fix video duration not being shown on thumbnails

Fix the video duration not being shown on thumbnails on the
https://danbooru.donmai.us/comments page.

BUG: this introduces duplicate HTML ids on the comments page. Post
thumbnails and post comment containers both have the same html ID.

app/javascript/src/styles/common/messages.scss

@@ -1,9 +1,10 @@
 div.list-of-messages {
   margin: 1em 0;
 
+  > * + * { margin-top: 2em; }
+
   article.message {
     display: flex;
-    margin-top: 2em;
     word-wrap: break-word;
     padding: 5px;
 

app/javascript/src/styles/common/utilities.scss

@@ -68,6 +68,7 @@ $spacer: 0.25rem; /* 4px */
 .align-top { vertical-align: top; }
 .align-middle { vertical-align: middle; }
 
+.flex-1 { flex: 1 1 0%; }
 .flex-auto { flex: 1 1 auto; }
 .items-center { align-items: center; }
 .justify-center { justify-content: center; }