R1:739bdeccabe7

Improve some Spaces behaviors

Summary:
Ref T8449. Try out some more subtle behaviors:

- Make the "Space" control part of the policy control, so the UI shows "Visible To: [Space][Policy]". I think this helps make the role of spaces more clear. It also makes them easier to implement.
- Don't show the default space in headers: instead, show nothing.
- If the user has access to only one space, pretend spaces don't exist (no edit controls, no header stuff).

This might be confusing, but I think most of…