Make showing all public spaces when the user has no Spaces configurable.

This came up with a site that has *loads* of Public spaces. By default, OA will show all public Spaces if the current user has no subscribed Spaces. This just created confusion. This change adds a configurable variable to disable this.

Ideally, there'd be some way to hook in and provide a list of default Spaces (maybe just 1 or 2 out of the "loads") but I couldn't come up with a clever way to do this... yet. :-)