MM-34549 shared channels; add users to channel that were already sync'd (#17361)

Fixes a bug and adds a feature for shared channels:
- The Bug: when creating new shared channels, users that had already been sync'd via another channel were not added to the new channel's member list, since the users were not sync'd again. This PR sync's users per channel.
- The Feature: support custom statuses

model/shared_channel.go

@@ -183,6 +183,7 @@ type SharedChannelRemoteStatus struct {
 type SharedChannelUser struct {
 	Id         string `json:"id"`
 	UserId     string `json:"user_id"`
+	ChannelId  string `json:"channel_id"`
 	RemoteId   string `json:"remote_id"`
 	CreateAt   int64  `json:"create_at"`
 	LastSyncAt int64  `json:"last_sync_at"`
@@ -202,6 +203,10 @@ func (scu *SharedChannelUser) IsValid() *AppError {
 		return NewAppError("SharedChannelUser.IsValid", "model.channel.is_valid.id.app_error", nil, "UserId="+scu.UserId, http.StatusBadRequest)
 	}
 
+	if !IsValidId(scu.ChannelId) {
+		return NewAppError("SharedChannelUser.IsValid", "model.channel.is_valid.id.app_error", nil, "ChannelId="+scu.ChannelId, http.StatusBadRequest)
+	}
+
 	if !IsValidId(scu.RemoteId) {
 		return NewAppError("SharedChannelUser.IsValid", "model.channel.is_valid.id.app_error", nil, "RemoteId="+scu.RemoteId, http.StatusBadRequest)
 	}