Adds mystery description fallback

If a message had no obvious text because it didn't have a content type
of post, use the mystery description as to not break the `postSnippet()`
function and give a valid snippet in the title.

src/views/index.js

@@ -711,7 +711,7 @@ exports.threadView = ({ messages }) => {
   const rootMessage = messages[0];
   const rootAuthorName = rootMessage.value.meta.author.name;
   const rootSnippet = postSnippet(
-    lodash.get(rootMessage, "value.content.text")
+    lodash.get(rootMessage, "value.content.text", i18n.mysteryDescription)
   );
   return template([`@${rootAuthorName}: `, rootSnippet], thread(messages));
 };