Docx writer: Create content types and document rels from scratch.

This fixes problems that arise when you edit the reference.docx
with Word.  Word tends to remove things from the `[Content_Types].xml`
and `word/_rels/document.xml.rels` files that are needed (e.g.
references to the `footnotes.xml` file and image default mime types).
So we regenerate these completely rather than taking them from
the `reference.docx`.

We also now encode mime types for each individual image rather
than using defaults.  This should allow us to handle a wider
range of image types.

This mostly addresses #414.  The only remaining issue I can see
is the issue of style IDs, which Word inexplicably changes in
some cases when the reference.docx is saved.  E.g.
`FootnoteReference` becomes `FootnoteReference1`.

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