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Tell developers to correctly nest calls to forum_print_post

Improve the format of the HTML representing a forum post.

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.

Because posts are now nested properly, the heading for each one refers to it's own article
and we don't need to generate arbitrary heading level tags depending on how deep the post
is nested.

Previously running the code to generate the download file
was very fagile. Now the code can be run anywhere without
problems occuring copying files.

Prefer to use freesans font for generated pdfs as it has the biggest charset support.