Update Slowvote voting methods to use sensible string constants

Summary: Ref T13682. Use API-friendly string constants instead of opaque integers in Slowvote voting methods.

Test Plan: Created, edited, and voted in polls with various voting methods. Examined database after migrations.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13682

Differential Revision: https://secure.phabricator.com/D21846

resources/sql/autopatches/20220525.slowvote.06.method-type.sql

···
+ALTER TABLE {$NAMESPACE}_slowvote.slowvote_poll
+  CHANGE method method VARCHAR(32) NOT NULL COLLATE {$COLLATE_TEXT};

resources/sql/autopatches/20220525.slowvote.07.method-value.sql

···
+UPDATE {$NAMESPACE}_slowvote.slowvote_poll
+  SET method = 'plurality' WHERE method = '0';
+
+UPDATE {$NAMESPACE}_slowvote.slowvote_poll
+  SET method = 'approval' WHERE method = '1';

src/applications/slowvote/constants/SlowvotePollVotingMethod.php

···
 final class SlowvotePollVotingMethod
   extends Phobject {
 
-  const METHOD_PLURALITY = 0;
-  const METHOD_APPROVAL = 1;
+  const METHOD_PLURALITY = 'plurality';
+  const METHOD_APPROVAL = 'approval';
 
   private $key;
 

src/applications/slowvote/controller/PhabricatorSlowvoteEditController.php

···
       $v_space = $request->getStr('spacePHID');
 
       if ($is_new) {
-        $poll->setMethod($request->getInt('method'));
+        $poll->setMethod($request->getStr('method'));
       }
 
       if (!strlen($v_question)) {

src/applications/slowvote/storage/PhabricatorSlowvotePoll.php

···
         'question' => 'text255',
         'responseVisibility' => 'text32',
         'shuffle' => 'bool',
-        'method' => 'uint32',
+        'method' => 'text32',
         'description' => 'text',
         'isClosed' => 'bool',
       ),

src/applications/slowvote/view/SlowvoteEmbedView.php

···
 
     $percent = sprintf('%d%%', $count ? 100 * $choices / $count : 0);
 
-    switch ($poll->getMethod()) {
+    $method = $poll->getMethod();
+    switch ($method) {
       case SlowvotePollVotingMethod::METHOD_PLURALITY:
         $status = pht('%s (%d / %d)', $percent, $choices, $count);
         break;
       case SlowvotePollVotingMethod::METHOD_APPROVAL:
         $status = pht('%s Approval (%d / %d)', $percent, $choices, $count);
+        break;
+      default:
+        $status = pht('Unknown ("%s")', $method);
         break;
     }