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_join_test.go

@@ -29,12 +29,12 @@ func TestJoinCommands(t *testing.T) {
 
 	channel3 := Client.Must(Client.CreateDirectChannel(user2.Id)).Data.(*model.Channel)
 
-	rs5 := Client.Must(Client.Command(channel0.Id, "/join "+channel2.Name, false)).Data.(*model.CommandResponse)
+	rs5 := Client.Must(Client.Command(channel0.Id, "/join "+channel2.Name)).Data.(*model.CommandResponse)
 	if !strings.HasSuffix(rs5.GotoLocation, "/"+team.Name+"/channels/"+channel2.Name) {
 		t.Fatal("failed to join channel")
 	}
 
-	rs6 := Client.Must(Client.Command(channel0.Id, "/join "+channel3.Name, false)).Data.(*model.CommandResponse)
+	rs6 := Client.Must(Client.Command(channel0.Id, "/join "+channel3.Name)).Data.(*model.CommandResponse)
 	if strings.HasSuffix(rs6.GotoLocation, "/"+team.Name+"/channels/"+channel3.Name) {
 		t.Fatal("should not have joined direct message channel")
 	}