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_logout_test.go

@@ -10,5 +10,5 @@ import (
 func TestLogoutTestCommand(t *testing.T) {
 	th := Setup().InitBasic()
 
-	th.BasicClient.Must(th.BasicClient.Command(th.BasicChannel.Id, "/logout", false))
+	th.BasicClient.Must(th.BasicClient.Command(th.BasicChannel.Id, "/logout"))
 }