PERF: Remove request for PM topic tracking state initiated from sidebar (#20554)

What is the problem?

When constructing the "Messages" section in Sidebar, we call
startTracking() on the pm-topic-tracking-state service in order to
get the counts for new/unread for the private message inboxes for each
user. However, this is unnecessary because the inboxes are in a
collapsed state by default in the sidebar and are only expanded when the
current route correspond to the inbox's route. Therefore, we can avoid
calling startTracking() on the pm-topic-tracking-state service until
an inbox's route is loaded. This allows us to cut out one extra request
to the server on page load and defer it until it is necessarily.

app/assets/javascripts/discourse/app/components/sidebar/user/messages-section.js

@@ -38,10 +38,6 @@ export default class SidebarUserMessagesSection extends Component {
       this._refreshSectionLinksDisplayState
     );
 
-    this.pmTopicTrackingState
-      .startTracking()
-      .then(this._refreshSectionLinkCounts);
-
     this._pmTopicTrackingStateKey = "messages-section";
 
     this.pmTopicTrackingState.onStateChange(

app/assets/javascripts/discourse/tests/acceptance/sidebar-user-messages-section-test.js

@@ -376,6 +376,10 @@ acceptance(
         "service:pm-topic-tracking-state"
       );
 
+      await click(
+        ".sidebar-section-messages .sidebar-section-link-group-messages-inbox.group1"
+      );
+
       await publishToMessageBus(pmTopicTrackingState.groupChannel(1), {
         topic_id: 1,
         message_type: "unread",
@@ -398,10 +402,6 @@ acceptance(
         },
       });
 
-      await click(
-        ".sidebar-section-messages .sidebar-section-link-group-messages-inbox.group1"
-      );
-
       assert.strictEqual(
         query(
           ".sidebar-section-messages .sidebar-section-link-group-messages-unread.group1"
@@ -449,6 +449,10 @@ acceptance(
         "service:pm-topic-tracking-state"
       );
 
+      await click(
+        ".sidebar-section-messages .sidebar-section-link-personal-messages-inbox"
+      );
+
       await publishToMessageBus(pmTopicTrackingState.userChannel(), {
         topic_id: 1,
         message_type: "unread",
@@ -460,10 +464,6 @@ acceptance(
         },
       });
 
-      await click(
-        ".sidebar-section-messages .sidebar-section-link-personal-messages-inbox"
-      );
-
       assert.strictEqual(
         query(
           ".sidebar-section-messages .sidebar-section-link-personal-messages-unread"