tagging: Unify the clickable tag link generation.

This ensures that clicking a tag anyhere on the page does the same (adds
or removes it from the current filter, depending on if it was in there
already).

This also ensures that both url and HTML escaping is consistently
applied to the generated links, preventing a duplicate escaping of
& that could occur in $current_filter before. It now makes use of
blosxom_html_escape instead of encoding only &.