Always produce HTML5 doctype and new attributes, but not new elements

According to caniuse.com, a significant fraction of Web users are
still using Internet Explorer versions that do not support HTML5
sectioning elements. However, claiming we're XHTML 1.0 Strict
means we can't use features invented in the last 12 years, even if
they degrade gracefully in older browsers (like the role and placeholder
attributes).

This means our output is no longer valid according to any particular
DTD. Real browsers and other non-validator user-agents have never
cared about DTD compliance anyway, so I don't think this is a real loss.
Now that we're always using HTML5, <base href> can be relative
Remove now-redundant test-cases for a non-default html5 setting
We no longer have a test for DTD-valid XHTML 1.0, but at least check well-formedness

This means that people can do XSLT nonsense if they want to.

The failures are currently marked TODO because not everything in the
docwiki is in fact well-formed.
Emit vestigial xmlns so people can still pass ikiwiki output through XSLT
Add classes to form in search results

This is needed to style it, for example to reduce the width of the text
input and prevent layout issues.
search: add more classes as a hook for CSS. Thanks, sajolida
Merge branch 'ready/html5'
core: generate HTML5 by default, but keep avoiding new elements like <section> that require specific browser support unless html5 is set to 1.