dwiki.tmpl: allow overrides to the page footer 'datecrumbs'

The page footer datecrumbs implement the standard untitled previous/next
links on blog entry pages. This can now be replaced with other options
such as proper previous/next links that show the entry titles, either by
putting your replacement into your own datecrumbs or by overriding this
with an empty datecrumbs.tmpl and doing the work in another template.