Plugin API hook for Shared Channel file attachment sync (#25874)

* option for auto inviting plugin to all shared channels.

* auto-invite remotes to shared channels when flag set

* fix unit test

* immediately ping new remotes; fix unique siteurl bug

* make i18n-extract

* fix translations

* plugin hooks for file attachments

* hook for profile image sync

* fix profile image sync

* fix unit test

---------

Co-authored-by: Mattermost Build <build@mattermost.com>

server/public/model/remote_cluster.go

@@ -174,8 +174,11 @@ func (rc *RemoteCluster) IsPlugin() bool {
 
 func (rc *RemoteCluster) GetSiteURL() string {
 	siteURL := rc.SiteURL
+	if strings.HasPrefix(siteURL, SiteURLPending) {
+		siteURL = "..."
+	}
 	if strings.HasPrefix(siteURL, SiteURLPending) || strings.HasPrefix(siteURL, SiteURLPlugin) {
-		siteURL = ""
+		siteURL = "plugin"
 	}
 	return siteURL
 }