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/applications/phriction/controller/diff/PhrictionDiffController.php

@@ -205,6 +205,12 @@ class PhrictionDiffController
     $document_id = $content->getDocumentID();
     $version = $content->getVersion();
 
+    if ($content->getChangeType() == PhrictionChangeType::CHANGE_DELETE) {
+      // Don't show an edit/revert button for changes which deleted the content
+      // since it's silly.
+      return null;
+    }
+
     if ($content->getID() == $current->getID()) {
       return phutil_render_tag(
         'a',