Bug #1066753 -- subtitle is severely broken (reported by Sam Steingold)

Bug #1066753 -- subtitle is severely broken (reported by Sam Steingold)

::PROBLEM
  In (X)HTML output, subtitle for the simplesect element generates an
  h2 head, no matter at what level in the sectioning hiearchy the
  simplesect occurs; end up with cases where a simplesect is
  output with (for example), an h4 title but a h2 subtitle

::CAUSE and FIX
  No template for simplesect/subtitle in the set of templates that
  call the section.subtitle named template, so stylesheets fell
  back to using the titlepage.mode template for subtitle

::FIX
  Added simplesect/subtitle to the set of templates that
  call the section.subtitle named template.

::AFFECTS
  Only affects (X)HTML output of simplesect/subtitle; nothing else