Clobbering refinement in AkomaNtoso import

We use a three-level section hierarchy for parliamentary debates: electoral term - session - sitting. There is one AN file per sitting and the files share up to two outer level sections. To import a file into an existing hierarchy and still have an opportunity to replace existing sections I propose the following two changes in AkomaNtoso import.

Clobbering works on all levels of section hierarchy now, not on the top-level section only.

There are three options for clobbering of sections with the same heading:

* `skip` to skip import of a section that already exists,
* `replace` to delete existing section and replace it with the imported one,
* `merge` to append content of the imported section to the existing one.

It might be better also to join respective command-line options into something like `--clobbering` with three allowed values `skip`, `replace` and `merge`. I just added third separate option `--merge-existing` to the existing two in sake of backwards compatibility.


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Yes, we do have some code to check that at least one of `num`, `heading`, `subheading` is always set for a Section, see https://github.com/mysociety/sayit/blob/master/speeches/models.py#L265-L268 - but that code isn't run by the importer currently, so can lead to the issue you describe. A PR currently in progress, #435, updates `title()` to be "untitled" if nothing else is provided, so that a slug will at least always be generated and that should fix this error.