R1:ef90007a2120

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…