docx: improve page break fidelity (#1631)
Page breaks can and often do occur within a paragraph. The full text of
the paragraph is attributed to the page (number) the paragraph starts
on.
Improve page-break fidelity such that a paragraph containing a
page-break is split into two elements, one containing the text before
the page-break and the other the text after. Emit the `PageBreak`
element between these two and assign the correct page-number (n and n+1
respectively) to the two textual elements.
This functionality is largely provided upstream by the new `python-docx`
v1.0.0 release (1.0.0 from 0.8.11 because it drops Python 2 support).
That version also makes obsolete the "include hyperlink text in
`Paragraph.text` monkey patch that we had maintained up to now. Remove
that monkey-patch.
