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.

app/import.go

@@ -865,7 +865,7 @@ func OldImportUser(team *model.Team, user *model.User) *model.User {
 			l4g.Error(utils.T("api.import.import_user.set_email.error"), cresult.Err)
 		}
 
-		if err := JoinUserToTeam(team, user); err != nil {
+		if err := JoinUserToTeam(team, user, ""); err != nil {
 			l4g.Error(utils.T("api.import.import_user.join_team.error"), err)
 		}