Add support for forum topics

Although the "module" sub-directory contains the file "forum.node_convert_behavior.inc", it is not implemented. The code appears to be an unfinished conversion from the Drupal 6 release, and as it stands doesn't work.

Without a working forum behaviour module, conversions to and from the forum_topic content type are incomplete and cause various user issues. Conversion from forum_topic causes the most problems.

I have re-worked the code in the forum behaviour module to do the right stuff. This includes dealing with ACL records if the forum_access module is installed.

I use Node Convert to archive old forum content into a different content type, and this process is now working correctly on my site. Also the occasional reverse conversion.