Use string constants, not integer constants, to represent task status internally

Summary:
Ref T1812. I think integer constants are going to be confusing and error prone for users to interact with. For example, because we use 0-5, adding a second "open" status like "needs verification" without disrupting the existing statuses would require users to define a status with, e.g., constant `6`, but order it between constants `0` and `1`. And if they later remove statuses, they need to avoid reusing existing constants.

Instead, use more manageable string constants like "open", "resolved", etc.

We must migrate three tables:

  - The task table itself, to update task status.
  - The transaction table, to update historic status changes.
  - The saved query table, to update saved queries which specify status sets.

Test Plan:
  - Saved a query with complicated status filters.
  - Ran migrations.
  - Looked at the query, at existing tasks, and at task transactions.
  - Forced migrations to run again to verify idempotentcy/safety.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T1812

Differential Revision: https://secure.phabricator.com/D8583

src/applications/maniphest/query/ManiphestTaskQuery.php

@@ -351,12 +351,12 @@ final class ManiphestTaskQuery
       case self::STATUS_OPEN:
         return qsprintf(
           $conn,
-          'status IN (%Ld)',
+          'status IN (%Ls)',
           ManiphestTaskStatus::getOpenStatusConstants());
       case self::STATUS_CLOSED:
         return qsprintf(
           $conn,
-          'status IN (%Ld)',
+          'status IN (%Ls)',
           ManiphestTaskStatus::getClosedStatusConstants());
       default:
         $constant = idx($map, $this->status);
@@ -365,7 +365,7 @@ final class ManiphestTaskQuery
         }
         return qsprintf(
           $conn,
-          'status = %d',
+          'status = %s',
           $constant);
     }
   }
@@ -374,7 +374,7 @@ final class ManiphestTaskQuery
     if ($this->statuses) {
       return qsprintf(
         $conn,
-        'status IN (%Ld)',
+        'status IN (%Ls)',
         $this->statuses);
     }
     return null;
@@ -826,8 +826,8 @@ final class ManiphestTaskQuery
       case self::GROUP_STATUS:
         $columns[] = array(
           'name' => 'task.status',
-          'value' => (int)$group_id,
-          'type' => 'int',
+          'value' => $group_id,
+          'type' => 'string',
         );
         break;
       case self::GROUP_PROJECT: