Provide Template file for forum topic(s)

Problem/Motivation

Forum topic markup is defined in PHP, not in a Twig template.

        $row[] = [
          'data' => [
            $topic->icon,
            [
              '#markup' => '<div class="forum__title"><div>' . $topic->title_link . '</div><div>' . $topic->submitted . '</div></div>',
            ],
          ],
          'class' => ['forum__topic'],
        ];

Steps to reproduce

Proposed resolution

Add a forum-topics.html.twig for the topics listup (/forum/x).

Remaining tasks

User interface changes

API changes

Data model changes