Add spacePHID infrastructure and implement in Paste

Summary:
Ref T8424. I'm using Paste as a testbed application because Spaces make some degree of sense for it but it's also flat/simple.

This doesn't do anything interesting or useful and mostly just making the next (more interesting) diff smaller.

Test Plan:

• Ran bin/storage upgrade -f.
• Browsed pastes.
• Created a paste.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T8424

Differential Revision: https://secure.phabricator.com/D13154