Display subforum listing on forumlist via template loop instead of PHP implode()

As was discussed here: http://tracker.phpbb.com/browse/PHPBB3-10901?focusedCommentId=37774&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-37774
There are currently two mechanisms for generating the subforum listing, which are both already present in functions_display.php. The way that is currently utilized is using implode() on ',' to create all the HTML on the PHP side and send it as a single template variable to the template.

The second way that is also already available is by assigning the subforum information to a template loop so that it can be handled on the template side instead. This allows individual styles to handle the subforum listing differently instead of all of them having to work with what is given to them from the PHP.

As such, this ticket is for retaining both mechanisms but simply changing what is done in the template files of subsilver and prosilver to use the template loop instead of the compiled template variable.