Support Spaces transactions

Summary:
Ref T8424. This adds crude integration with Paste's edit/view workflows: you can change the space a Paste appears in, see transactions, and get a policy callout.

Lots of rough edges and non-obviousness but it pretty much works.

Test Plan:

• Created and updated Pastes.
• Moved them between spaces, saw policy effects.
• Read transactions.
• Looked at feed.
• Faked query to return no spaces, saw control and other stuff vanish.
• Faked query to return no spaces, created pastes.
• Tried to submit bad values and got errors.

Reviewers: chad, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T8424

Differential Revision: https://secure.phabricator.com/D13159