PLT-1598 Slash command works in RHS (#4367)

* PLT-1598 Slash command works in RHS

* fix UserProfile in the reply for Slash Command

* fix some problem about the system messages in RHS
* system message in RHS isn't displayed as comment for root message

* remove status indicator for system message in RHS

* system message in RHS is colored to grey

* system messages don't count as commented post

* fix bug about cleaning draft in RHS

* remove unnecessary function

* implement new model for executing command

api/command_echo.go

@@ -39,7 +39,7 @@ func (me *EchoProvider) GetCommand(c *Context) *model.Command {
 	}
 }
 
-func (me *EchoProvider) DoCommand(c *Context, channelId string, message string) *model.CommandResponse {
+func (me *EchoProvider) DoCommand(c *Context, args *model.CommandArgs, message string) *model.CommandResponse {
 	maxThreads := 100
 
 	delay := 0
@@ -75,7 +75,9 @@ func (me *EchoProvider) DoCommand(c *Context, channelId string, message string)
 	go func() {
 		defer func() { <-echoSem }()
 		post := &model.Post{}
-		post.ChannelId = channelId
+		post.ChannelId = args.ChannelId
+		post.RootId = args.RootId
+		post.ParentId = args.ParentId
 		post.Message = message
 		post.UserId = c.Session.UserId