message_events: Avoid full rerender when marking unread.

Previously, when you did a "mark as unread from here" operation, we triggered 
a full rerender of both the recent topics view and the message feed. This was 
needlessly expensive, on and a large server with a somewhat busy CPU from 
other applications, can cause a visible lag, even when the message feed that 
you're looking at only has like 3 messages in it.

Improve this by passing the set of modified messages to the rerender.

There's likely further improvements to be made here -- we shouldn't need to
do more than toggle the unread markers -- but this should be good enough to
eliminate the visible lag.

Fixes #24263.

web/src/message_live_update.js

@@ -8,6 +8,19 @@ export function rerender_messages_view() {
     }
 }
 
+export function rerender_messages_view_by_message_ids(message_ids) {
+    const messages_to_render = [];
+    for (const id of message_ids) {
+        const message = message_store.get(id);
+        if (message !== undefined) {
+            messages_to_render.push(message);
+        }
+    }
+    for (const list of message_lists.all_rendered_message_lists()) {
+        list.view.rerender_messages(messages_to_render);
+    }
+}
+
 function rerender_messages_view_for_user(user_id) {
     for (const list of message_lists.all_rendered_message_lists()) {
         const messages = list.data.get_messages_sent_by_user(user_id);

web/src/unread_ops.js

@@ -329,7 +329,7 @@ export function process_unread_messages_event({message_ids, message_details}) {
         The main downside of doing a full rerender is that it can be
         user-visible in the form of users' avatars flickering.
     */
-    message_live_update.rerender_messages_view();
+    message_live_update.rerender_messages_view_by_message_ids(message_ids);
     recent_topics_ui.complete_rerender();
 
     if (