Add spacePHID infrastructure and implement in Paste
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.
- Ran bin/storage upgrade -f.
- Browsed pastes.
- Created a paste.
Reviewed By: btrahan
Maniphest Tasks: T8424
Differential Revision: https://secure.phabricator.com/D13154