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

src/applications/spaces/interface/PhabricatorSpacesInterface.php

@@ -1,3 +1,18 @@
 <?php
 
-interface PhabricatorSpacesInterface extends PhabricatorPHIDInterface {}
+interface PhabricatorSpacesInterface extends PhabricatorPHIDInterface {
+
+  public function getSpacePHID();
+
+}
+
+// TEMPLATE IMPLEMENTATION /////////////////////////////////////////////////////
+
+/* -(  PhabricatorSpacesInterface  )----------------------------------------- */
+/*
+
+  public function getSpacePHID() {
+    return $this->spacePHID;
+  }
+
+*/