Take section number position into account on import?

[goose-life]

Currently the hierarchicalizer doesn't take the section_number_position on the importer into account, and works hard to guess at section headings in either the pre or post position — and doesn't always get it right.

Individual documents are consistent in the placement of their section headings, and we have an existing mechanism for allowing the user to choose the position on the import screen.

So let's take the section_number_position into account instead, and allow that to inform how section headings are identified in the pipeline.

https://edit.laws.africa/documents/10424/import-quality sections 9, 16