Since 0.18, Element.traverse() returns an iterator instead of
intermediate object.  As a result, the return value is always considered
as truthy value.  And it becomes fragile when the caller modifies the
doctree on the loop.

Support docutils-0.18: allow PreBibliographic nodes before docinfo

Since 0.18, `meta` directive inserts meta node into the top of the
document.  It confuses MetadataCollector.

This allows doctree contains PreBibliographic nodes just before docinfo
node.

Since docutils-0.18, auto_id_prefix setting will be changed to `'%'`
from `'id'`.  To keep backward compatibility of node IDs, this sets
`'id'` to settings explicitly.

embed_images:
    HTML Builder always use non-embeded (linked) images.

section_self_link:
    HTML Builder controls hyperlink generation for each section via
    `html_permalinks` option.

refactor: Make the app argument for BuildEnvironment required

At present, some attributes of env object are considered as optional
because they have been initialized by None on the constructor.  But
they have always been fullfilled actually.

To be clear the type hints of the env object, this makes the `app`
argument for the BuildEnvironment class required.  It can ensure the
attributes of env object are not optional.

Also change any docutils.sourceforge.net link to io
top level domain.