Refactor Get/Create Direct Channel into one function (#9867)

* refactor GetDirectChannel and CreateDirectChannel in one function

* remove CreateDirectChannel plugin api and update GetDirectChannel and GetGroupChannel plugin api

* update tests

plugin/api.go

@@ -52,11 +52,6 @@ type API interface {
 	// CreateUser creates a user.
 	CreateUser(user *model.User) (*model.User, *model.AppError)
 
-	// CreateDirectChannel creates a Direct channel.
-	//
-	// Minimum server version: 5.6
-	CreateDirectChannel(userId1 string, userId2 string) (*model.Channel, *model.AppError)
-
 	// DeleteUser deletes a user.
 	DeleteUser(userId string) *model.AppError
 
@@ -196,9 +191,11 @@ type API interface {
 	GetChannelStats(channelId string) (*model.ChannelStats, *model.AppError)
 
 	// GetDirectChannel gets a direct message channel.
+	// If the channel does not exist it will create it.
 	GetDirectChannel(userId1, userId2 string) (*model.Channel, *model.AppError)
 
 	// GetGroupChannel gets a group message channel.
+	// If the channel does not exist it will create it.
 	GetGroupChannel(userIds []string) (*model.Channel, *model.AppError)
 
 	// UpdateChannel updates a channel.