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)

services/slackimport/slackimport.go

@@ -85,7 +85,7 @@ type Actions struct {
 	UpdateActive           func(*model.User, bool) (*model.User, *model.AppError)
 	AddUserToChannel       func(*model.User, *model.Channel) (*model.ChannelMember, *model.AppError)
 	JoinUserToTeam         func(*model.Team, *model.User, string) *model.AppError
-	CreateDirectChannel    func(string, string) (*model.Channel, *model.AppError)
+	CreateDirectChannel    func(string, string, ...model.ChannelOption) (*model.Channel, *model.AppError)
 	CreateGroupChannel     func([]string) (*model.Channel, *model.AppError)
 	CreateChannel          func(*model.Channel, bool) (*model.Channel, *model.AppError)
 	DoUploadFile           func(time.Time, string, string, string, string, []byte) (*model.FileInfo, *model.AppError)