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

src/applications/spaces/application/PhabricatorSpacesApplication.php

@@ -15,7 +15,7 @@ final class PhabricatorSpacesApplication extends PhabricatorApplication {
   }
 
   public function getFontIcon() {
-    return 'fa-compass';
+    return 'fa-th-large';
   }
 
   public function getTitleGlyph() {