Move away from the "andForget" style of function (#3046)

This is the second and last part of the refactoring.
First part is documented here:
https://github.com/mattermost/platform/pull/3043

api/team.go

@@ -274,7 +274,7 @@ func JoinUserToTeam(team *model.Team, user *model.User) *model.AppError {
 	InvalidateCacheForUser(user.Id)
 
 	// This message goes to every channel, so the channelId is irrelevant
-	PublishAndForget(model.NewMessage("", "", user.Id, model.ACTION_NEW_USER))
+	go Publish(model.NewMessage("", "", user.Id, model.ACTION_NEW_USER))
 
 	return nil
 }