Allow Phriction documents to be deleted

Summary:
  - Add a "delete" operation. Delete is just a special edit which removes the
page from indexes and shows a notice that the document has been deleted.
  - When a user deletes all the content on a page, treat it as a delete.
  - When a conduit call deletes all the content on a page, treat it as a delete.
  - Add page status to Conduit.
  - Add change type field to history.
  - Added a couple of constants to support a future 'move' change, which would
move content from one document to another.

Test Plan:
  - Verified deleted pages vanish from the document index (and restoring them
puts them back).
  - Verified deleted pages show "This page has been deleted...".
  - Created, edited and deleted a document via Conduit.
  - Deleted pages via "delete" button.
  - Deleted pages via editing content to nothing.

Reviewers: btrahan, jungejason

Reviewed By: btrahan

CC: skrul, aran, btrahan, epriestley

Maniphest Tasks: T680

Differential Revision: 1230

src/aphront/default/configuration/AphrontDefaultApplicationConfiguration.php

@@ -365,6 +365,7 @@ class AphrontDefaultApplicationConfiguration
         'history/(?P<slug>.+/)$'  => 'PhrictionHistoryController',
 
         'edit/(?:(?P<id>\d+)/)?$' => 'PhrictionEditController',
+        'delete/(?P<id>\d+)/$'    => 'PhrictionDeleteController',
 
         'preview/$' => 'PhrictionDocumentPreviewController',
         'diff/(?P<id>\d+)/$' => 'PhrictionDiffController',