Forum: Improve structure of forum post HTML

HTML structure of a forum post can be improved.

Improvements:

1. Use <article> tag to mark it correctly.
2. Use <time> tag to include the real datetime information.
3. Use <address> tag to specify who and when the post came from.
4. Use heading level tags to mark the subject of each post.
5. Properly nest reply posts within their parent <article>
6. Use a header tab for each article to split the metadata from the content.
7. Use CSS to separate links, not text content.