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_shortcuts_test.go

@@ -14,12 +14,12 @@ func TestShortcutsCommand(t *testing.T) {
 	Client := th.BasicClient
 	channel := th.BasicChannel
 
-	rs := Client.Must(Client.Command(channel.Id, "/shortcuts ", false)).Data.(*model.CommandResponse)
+	rs := Client.Must(Client.Command(channel.Id, "/shortcuts ")).Data.(*model.CommandResponse)
 	if !strings.Contains(rs.Text, "CTRL") {
 		t.Fatal("failed to display shortcuts")
 	}
 
-	rs = Client.Must(Client.Command(channel.Id, "/shortcuts mac", false)).Data.(*model.CommandResponse)
+	rs = Client.Must(Client.Command(channel.Id, "/shortcuts mac")).Data.(*model.CommandResponse)
 	if !strings.Contains(rs.Text, "CMD") {
 		t.Fatal("failed to display Mac shortcuts")
 	}