MM-27493 Shared channels (MVP) (#17301)

Remote Cluster Service
- provides ability for multiple Mattermost cluster instances to create a trusted connection with each other and exchange messages
- trusted connections are managed via slash commands (for now)
- facilitates features requiring inter-cluster communication, such as Shared Channels
Shared Channels Service
- provides ability to shared channels between one or more Mattermost cluster instances (using trusted connection)
- sharing/unsharing of channels is managed via slash commands (for now)

app/upload.go

@@ -253,6 +253,10 @@ func (a *App) UploadData(us *model.UploadSession, rd io.Reader) (*model.FileInfo
 
 	info.CreatorId = us.UserId
 	info.Path = us.Path
+	info.RemoteId = model.NewString(us.RemoteId)
+	if us.ReqFileId != "" {
+		info.Id = us.ReqFileId
+	}
 
 	// run plugins upload hook
 	if err := a.runPluginsHook(info, file); err != nil {