recent topics: Handle muted user avatars in participants.

We use styling same as that of the number shown for overflow
(e.g "+3") which has a gray background.
To keep these avatars up-to-date, we need to rerender the
recent topics view after receiving a `muted_users` event.

Also update the user documentation to mention this detail.
We deliberately avoid mentioning "recent topics" because
this applies to mobile too.

frontend_tests/node_tests/people.js

@@ -681,9 +681,11 @@ test_people("message_methods", () => {
         "https://secure.gravatar.com/avatar/6dbdd7946b58d8b11351fcb27e5cdd55?d=identicon&s=50",
     );
 
+    muting.add_muted_user(30);
     assert.deepEqual(people.sender_info_for_recent_topics_row([30]), [
         {
             avatar_url_small: "/avatar/30&s=50",
+            is_muted: true,
             email: "me@example.com",
             full_name: me.full_name,
             is_admin: false,