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/slackimport.go

@@ -160,7 +160,7 @@ func SlackAddUsers(teamId string, slackusers []SlackUser, log *bytes.Buffer) map
 		if result := <-Srv.Store.User().GetByEmail(email); result.Err == nil {
 			existingUser := result.Data.(*model.User)
 			addedUsers[sUser.Id] = existingUser
-			if err := JoinUserToTeam(team, addedUsers[sUser.Id]); err != nil {
+			if err := JoinUserToTeam(team, addedUsers[sUser.Id], ""); err != nil {
 				log.WriteString(utils.T("api.slackimport.slack_add_users.merge_existing_failed", map[string]interface{}{"Email": existingUser.Email, "Username": existingUser.Username}))
 			} else {
 				log.WriteString(utils.T("api.slackimport.slack_add_users.merge_existing", map[string]interface{}{"Email": existingUser.Email, "Username": existingUser.Username}))