Make consistent creating of the authas form

* prefered is to pass in authas => 1 or authas => { show_all => 1 } (or
  whatever args) to controller(). For foundation pages, use
  $rv->{authas_form}, where the authas is wrapped in an actual form. For
  non-foundation pages, use $rv->{authas_html} which only has the select
  HTML. This means that we don't have to worry about checking for
  authas, so long as we make sure to use $rv->{u} versus $rv->{remote}
  where it matters

* remove now-obsolete plugin authas function