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_loadtest.go

@@ -84,7 +84,8 @@ func (me *LoadTestProvider) GetCommand(c *Context) *model.Command {
 	}
 }
 
-func (me *LoadTestProvider) DoCommand(c *Context, channelId string, message string) *model.CommandResponse {
+func (me *LoadTestProvider) DoCommand(c *Context, args *model.CommandArgs, message string) *model.CommandResponse {
+	channelId := args.ChannelId
 
 	//This command is only available when EnableTesting is true
 	if !utils.Cfg.ServiceSettings.EnableTesting {