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.

api/apitestlib.go

@@ -163,7 +163,7 @@ func (me *TestHelper) CreateUser(client *model.Client) *model.User {
 func LinkUserToTeam(user *model.User, team *model.Team) {
 	utils.DisableDebugLogForTest()
 
-	err := app.JoinUserToTeam(team, user)
+	err := app.JoinUserToTeam(team, user, "")
 	if err != nil {
 		l4g.Error(err.Error())
 		l4g.Close()