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.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T8493

Differential Revision: https://secure.phabricator.com/D13414