Replace !placeholder with @placeholder in comment module

We missed a couple of replacements:

CommentFieldAccessTest::testAccessToAdministrativeFields()
core/modules/comment/src/Tests/CommentFieldAccessTest.php:
Line 263:
// Check create-only fields.
    foreach ($this->createOnlyFields as $field) {
      // Check view operation.
      foreach ($permutations as $set) {
        $may_view = $set['comment']->{$field}->access('view', $set['user']);
        $may_update = $set['comment']->{$field}->access('edit', $set['user']);
        $this->assertEqual($may_view, $field != 'hostname' && ($set['user']->hasPermission('administer comments') ||
            ($set['comment']->isPublished() && $set['user']->hasPermission('access comments'))), SafeMarkup::format('User @user !state view field !field on comment @comment', [
          '@user' => $set['user']->getUsername(),
          '!state' => $may_view ? 'can' : 'cannot',
          '@comment' => $set['comment']->getSubject(),
          '!field' => $field,
        ]));
        $this->assertEqual($may_update, $set['user']->hasPermission('post comments') && $set['comment']->isNew(), SafeMarkup::format('User @user !state update field !field on comment @comment', [
          '@user' => $set['user']->getUsername(),
          '!state' => $may_update ? 'can' : 'cannot',
          '@comment' => $set['comment']->getSubject(),
          '!field' => $field,
        ]));
      }
    }

Well that simplifies manual testing a bit. The only location that is not a test is in the meta information on a comment display. Since this specific text is visually hidden, including only a grab of the source, which matches exactly before the patch:

Comment #4 has been addressed and I don't see any other occurrences of !placeholder; all changes are in the scope of this module.
Updating IS to indicate separation of hook_help() from this issue. Good to go.

Wow, that last image was too scaled to be legible. Trying again:

Still RTBC

Closing this, splitting by module was not the ideal approach to removing !placeholder. Marking as duplicate of #2506427: [meta] !placeholder causes strings to be escaped and makes the sanitization API harder to understand, since the chosen approach is / will be outlined there, please refer to it for any additional action.