When a field isn't lockable, just freeze the lock status instead of removing any lock

Summary:
See downstream issue here: <https://phabricator.wikimedia.org/T150992>

In at least one case (project milestones) we have a locked, non-lockable field. This means "this is locked, and you can't change the fact that it is locked".

At least for now, preserve this behavior.

Test Plan: Created a new milestone of an existing project. This worked correctly with the patch.

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D16895

src/applications/transactions/storage/PhabricatorEditEngineConfiguration.php

···
       switch (idx($locks, $key)) {
         case self::LOCK_LOCKED:
           $field->setIsHidden(false);
-          $field->setIsLocked(true);
+          if ($field->getIsLockable()) {
+            $field->setIsLocked(true);
+          }
           break;
         case self::LOCK_HIDDEN:
           $field->setIsHidden(true);
-          $field->setIsLocked(false);
+          if ($field->getIsLockable()) {
+            $field->setIsLocked(false);
+          }
           break;
         case self::LOCK_VISIBLE:
           $field->setIsHidden(false);
-          $field->setIsLocked(false);
+          if ($field->getIsLockable()) {
+            $field->setIsLocked(false);
+          }
           break;
         default:
           // If we don't have an explicit value, don't make any adjustments.
           break;
-      }
-
-      // If the field isn't lockable, remove any lock we applied.
-      if (!$field->getIsLockable()) {
-        $field->setIsLocked(false);
       }
     }