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)

store/sqlstore/user_store.go

@@ -53,7 +53,7 @@ func newSqlUserStore(sqlStore *SqlStore, metrics einterfaces.MetricsInterface) s
 	// note: we are providing field names explicitly here to maintain order of columns (needed when using raw queries)
 	us.usersQuery = us.getQueryBuilder().
 		Select("u.Id", "u.CreateAt", "u.UpdateAt", "u.DeleteAt", "u.Username", "u.Password", "u.AuthData", "u.AuthService", "u.Email", "u.EmailVerified", "u.Nickname", "u.FirstName", "u.LastName", "u.Position", "u.Roles", "u.AllowMarketing", "u.Props", "u.NotifyProps", "u.LastPasswordUpdate", "u.LastPictureUpdate", "u.FailedAttempts", "u.Locale", "u.Timezone", "u.MfaActive", "u.MfaSecret",
-			"b.UserId IS NOT NULL AS IsBot", "COALESCE(b.Description, '') AS BotDescription", "COALESCE(b.LastIconUpdate, 0) AS BotLastIconUpdate").
+			"b.UserId IS NOT NULL AS IsBot", "COALESCE(b.Description, '') AS BotDescription", "COALESCE(b.LastIconUpdate, 0) AS BotLastIconUpdate", "u.RemoteId").
 		From("Users u").
 		LeftJoin("Bots b ON ( b.UserId = u.Id )")
 
@@ -73,6 +73,7 @@ func newSqlUserStore(sqlStore *SqlStore, metrics einterfaces.MetricsInterface) s
 		table.ColMap("NotifyProps").SetMaxSize(2000)
 		table.ColMap("Locale").SetMaxSize(5)
 		table.ColMap("MfaSecret").SetMaxSize(128)
+		table.ColMap("RemoteId").SetMaxSize(26)
 		table.ColMap("Position").SetMaxSize(128)
 		table.ColMap("Timezone").SetMaxSize(256)
 	}
@@ -101,7 +102,7 @@ func (us SqlUserStore) createIndexesIfNotExists() {
 }
 
 func (us SqlUserStore) Save(user *model.User) (*model.User, error) {
-	if user.Id != "" {
+	if user.Id != "" && !user.IsRemote() {
 		return nil, store.NewErrInvalidInput("User", "id", user.Id)
 	}
 
@@ -358,7 +359,7 @@ func (us SqlUserStore) Get(ctx context.Context, id string) (*model.User, error)
 		&user.Nickname, &user.FirstName, &user.LastName, &user.Position, &user.Roles,
 		&user.AllowMarketing, &props, &notifyProps, &user.LastPasswordUpdate, &user.LastPictureUpdate,
 		&user.FailedAttempts, &user.Locale, &timezone, &user.MfaActive, &user.MfaSecret,
-		&user.IsBot, &user.BotDescription, &user.BotLastIconUpdate)
+		&user.IsBot, &user.BotDescription, &user.BotLastIconUpdate, &user.RemoteId)
 	if err != nil {
 		if err == sql.ErrNoRows {
 			return nil, store.NewErrNotFound("User", id)
@@ -727,7 +728,7 @@ func (us SqlUserStore) GetAllProfilesInChannel(ctx context.Context, channelID st
 	for rows.Next() {
 		var user model.User
 		var props, notifyProps, timezone []byte
-		if err = rows.Scan(&user.Id, &user.CreateAt, &user.UpdateAt, &user.DeleteAt, &user.Username, &user.Password, &user.AuthData, &user.AuthService, &user.Email, &user.EmailVerified, &user.Nickname, &user.FirstName, &user.LastName, &user.Position, &user.Roles, &user.AllowMarketing, &props, &notifyProps, &user.LastPasswordUpdate, &user.LastPictureUpdate, &user.FailedAttempts, &user.Locale, &timezone, &user.MfaActive, &user.MfaSecret, &user.IsBot, &user.BotDescription, &user.BotLastIconUpdate); err != nil {
+		if err = rows.Scan(&user.Id, &user.CreateAt, &user.UpdateAt, &user.DeleteAt, &user.Username, &user.Password, &user.AuthData, &user.AuthService, &user.Email, &user.EmailVerified, &user.Nickname, &user.FirstName, &user.LastName, &user.Position, &user.Roles, &user.AllowMarketing, &props, &notifyProps, &user.LastPasswordUpdate, &user.LastPictureUpdate, &user.FailedAttempts, &user.Locale, &timezone, &user.MfaActive, &user.MfaSecret, &user.IsBot, &user.BotDescription, &user.BotLastIconUpdate, &user.RemoteId); err != nil {
 			return nil, errors.Wrap(err, "failed to scan values from rows into User entity")
 		}
 		if err = json.Unmarshal(props, &user.Props); err != nil {