Drive query ordering and paging more cohesively

Summary:
Ref T7803. Ordering and paging are inherently intertwined, but they often aren't driven by the same data right now.

Start driving them through the same data:

  - `getOrderableColumns()` defines orderable and pageable columns.
  - `getPagingValueMap()` reads values from a cursor.

This is generally sufficient to implement both paging and ordering.

Also, add some more sanity checks to try to curtail the number of ambiguous/invalid orderings applications produce, since these cause subtle/messy bugs.

Test Plan:
  - Paged through pastes and a few other object types.
  - Intentionally changed defaults to be invalid and hit some of the errors.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7803

Differential Revision: https://secure.phabricator.com/D12355

src/infrastructure/query/order/PhabricatorQueryOrderVector.php

@@ -72,7 +72,7 @@ final class PhabricatorQueryOrderVector
             'Order vector "%s" specifies order "%s" twice. Each component '.
             'of an ordering must be unique.',
             implode(', ', $vector),
-            $item));
+            $item->getOrderKey()));
       }
 
       $items[$item->getOrderKey()] = $item;
@@ -84,6 +84,14 @@ final class PhabricatorQueryOrderVector
     return $obj;
   }
 
+  public function getAsString() {
+    $scalars = array();
+    foreach ($this->items as $item) {
+      $scalars[] = $item->getAsScalar();
+    }
+    return implode(', ', $scalars);
+  }
+
 
 /* -(  Iterator Interface  )------------------------------------------------- */