R1:fcb35a55fd23

Support Spaces in Diffusion

Summary:
Ref T8493. Diffusion is probably the strongest upstream use case we have for Spaces right now, so I want to get us on it to kick the tires a bit.

Small amount of hackiness around the multi-page form thing but it shouldn't create any problems.

Test Plan:
- Created a new repo.
- Edited a repo.
- Tried invalid edits, saw value preserved.
- Viewed edit full detail screen, saw space info.
- Viewed repo detail view, saw space.
- Viewed repo list view, saw space.