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.
Use archive_dir from params in latest_link.
Use $params in get_archive_location.
Only support arrays in file_not_found.
Don't display link to latest version in error pages.
Set build docs version to null.

Which is accurate, and removes the need for a special case. If boost
build needs special behaviour, that could be set by adding an extra
entry to `$params`.
Merge branch 'beta'