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_invite_people_test.go

@@ -14,17 +14,17 @@ func TestInvitePeopleCommand(t *testing.T) {
 	Client := th.BasicClient
 	channel := th.BasicChannel
 
-	r1 := Client.Must(Client.Command(channel.Id, "/invite_people test@example.com", false)).Data.(*model.CommandResponse)
+	r1 := Client.Must(Client.Command(channel.Id, "/invite_people test@example.com")).Data.(*model.CommandResponse)
 	if r1 == nil {
 		t.Fatal("Command failed to execute")
 	}
 
-	r2 := Client.Must(Client.Command(channel.Id, "/invite_people test1@example.com test2@example.com", false)).Data.(*model.CommandResponse)
+	r2 := Client.Must(Client.Command(channel.Id, "/invite_people test1@example.com test2@example.com")).Data.(*model.CommandResponse)
 	if r2 == nil {
 		t.Fatal("Command failed to execute")
 	}
 
-	r3 := Client.Must(Client.Command(channel.Id, "/invite_people", false)).Data.(*model.CommandResponse)
+	r3 := Client.Must(Client.Command(channel.Id, "/invite_people")).Data.(*model.CommandResponse)
 	if r3 == nil {
 		t.Fatal("Command failed to execute")
 	}