Prep for MS Teams plugin API (#25565)

* - columns added to ShareChannelRemotes: lastpostcreateat, lastpostupdateat
- SyncMsg and SyncResponse moved to `model` package
- field added to RemoteCluster struct: PluginID

* sync new posts before updated posts to ensure post order in MS Teams

* add plugid to remoteclusters table and store

* don't sync history by default

server/platform/services/sharedchannel/service.go

@@ -23,7 +23,7 @@ const (
 	TopicChannelInvite           = "sharedchannel_invite"
 	TopicUploadCreate            = "sharedchannel_upload"
 	MaxRetries                   = 3
-	MaxPostsPerSync              = 12 // a bit more than one typical screenfull of posts
+	MaxPostsPerSync              = 50 // a bit more than 4 typical screenfulls of posts
 	MaxUsersPerSync              = 25
 	NotifyRemoteOfflineThreshold = time.Second * 10
 	NotifyMinimumDelay           = time.Second * 2