PLT-6198: Use added to channel system message on default channels. (#5969)

Use a different sytem message when a user was added to a default channel
by someone else than when they joined themselves.

cmd/platform/team.go

@@ -154,7 +154,7 @@ func addUserToTeam(team *model.Team, user *model.User, userArg string) {
 		CommandPrintErrorln("Can't find user '" + userArg + "'")
 		return
 	}
-	if err := app.JoinUserToTeam(team, user); err != nil {
+	if err := app.JoinUserToTeam(team, user, ""); err != nil {
 		CommandPrintErrorln("Unable to add '" + userArg + "' to " + team.Name)
 	}
 }