Pass full $params array to 'display_template'.

The 'display_template' mechanism is pretty rubbish. There's only ever
one template, and it's probably not flexible enough to ever be of use.
Might be best to just remove it.
Move latest_link into boost_archive.php.

It's a display function rather than a filter function.
Add latest_link to more pages.
Fix latest_link for directories.