Add "Date Updated" query fields for Maniphest

Summary:
Fixes T4637.

  - We already allow you to order by this column but don't have a key on it. Add one.
  - Expose UI for querying on ranges.

Test Plan:
  - Ran some queries, got reasonable-looking results and no table scans.

Reviewers: btrahan, bigo

Reviewed By: bigo

Subscribers: bigo, epriestley

Maniphest Tasks: T4637

Differential Revision: https://secure.phabricator.com/D8557

resources/sql/autopatches/20140317.mupdatedkey.sql

@@ -0,0 +1,2 @@
+ALTER TABLE {$NAMESPACE}_maniphest.maniphest_task
+  ADD KEY `key_dateModified` (dateModified);