templates: add struct/ template cascade for full-title previous/next blog links

test/templates/struct/datecrumbs-full.tmpl and its two dependent templates
are an alternate to struct/datecrumbs.tmpl that generates previous and next
entry links that use the titles of their respective entries. They do this in
a <table> because I am evil or something.
Add better use of HTTP error codes

Previously DWiki almost always generated 404 errors, with the occasional
403 mixed in. Now we generate a more correct and wider variety
(including some potentially questionable choices, but that's for
later). This has added a new context variable, :error:code, for the
(string) HTTP error status during template-level error processing.