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_msg_test.go

@@ -21,12 +21,12 @@ func TestMsgCommands(t *testing.T) {
 	Client.Must(Client.CreateDirectChannel(user2.Id))
 	Client.Must(Client.CreateDirectChannel(user3.Id))
 
-	rs1 := Client.Must(Client.Command("", "/msg "+user2.Username, false)).Data.(*model.CommandResponse)
+	rs1 := Client.Must(Client.Command("", "/msg "+user2.Username)).Data.(*model.CommandResponse)
 	if !strings.HasSuffix(rs1.GotoLocation, "/"+team.Name+"/channels/"+user1.Id+"__"+user2.Id) && !strings.HasSuffix(rs1.GotoLocation, "/"+team.Name+"/channels/"+user2.Id+"__"+user1.Id) {
 		t.Fatal("failed to create direct channel")
 	}
 
-	rs2 := Client.Must(Client.Command("", "/msg "+user3.Username+" foobar", false)).Data.(*model.CommandResponse)
+	rs2 := Client.Must(Client.Command("", "/msg "+user3.Username+" foobar")).Data.(*model.CommandResponse)
 	if !strings.HasSuffix(rs2.GotoLocation, "/"+team.Name+"/channels/"+user1.Id+"__"+user3.Id) && !strings.HasSuffix(rs2.GotoLocation, "/"+team.Name+"/channels/"+user3.Id+"__"+user1.Id) {
 		t.Fatal("failed to create second direct channel")
 	}
@@ -34,7 +34,7 @@ func TestMsgCommands(t *testing.T) {
 		t.Fatalf("post did not get sent to direct message")
 	}
 
-	rs3 := Client.Must(Client.Command("", "/msg "+user2.Username, false)).Data.(*model.CommandResponse)
+	rs3 := Client.Must(Client.Command("", "/msg "+user2.Username)).Data.(*model.CommandResponse)
 	if !strings.HasSuffix(rs3.GotoLocation, "/"+team.Name+"/channels/"+user1.Id+"__"+user2.Id) && !strings.HasSuffix(rs3.GotoLocation, "/"+team.Name+"/channels/"+user2.Id+"__"+user1.Id) {
 		t.Fatal("failed to go back to existing direct channel")
 	}