Make sure stream created is public by default
Make sure stream created is public by default

This makes the behavior consistent with the web app.

The change affects two concepts that both benefit from it:

* the nullObject should have had `invite_only` set to `false` anyway
* new streams will now be created as 'public'

vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile