MM-57152: Get webconn count from the whole cluster (#26813)

We were setting the user status to offline without
checking for connections on other nodes in a cluster.

Now we implement a request-response mechanism for the whole
cluster and we check that before setting a user to offline.

https://mattermost.atlassian.net/browse/MM-57153

```release-note
Fix a bug where the user status would incorrectly be set to offline
without checking for connections in other nodes in an HA cluster.
```

Co-authored-by: Ibrahim Serdar Acikgoz <serdaracikgoz86@gmail.com>
Co-authored-by: Mattermost Build <build@mattermost.com>
Co-authored-by: Ben Schumacher <ben.schumacher@mattermost.com>

server/channels/app/platform/busy_test.go

@@ -149,3 +149,6 @@ func (c *ClusterMock) ConfigChanged(previousConfig *model.Config, newConfig *mod
 	return nil
 }
 func (c *ClusterMock) HealthScore() int { return 0 }
+func (c *ClusterMock) WebConnCountForUser(userID string) (int, *model.AppError) {
+	return 0, nil
+}