Install PyMuPDF under ./dangerzone/vendor, right before we build the
.deb package. We vendor PyMuPDF just for Debian, since the provided
versions don't have OCR support enabled.
Currently, we don't use PyMuPDf on the host, but this will change once
we fully implement the on-host conversion feature.
Refs #625
As a result, a new `debian` folder is now living in the repository.
Debian packaging is now done manually rather than using tools that do
the heavy-lifting for us.
The `build-deb.py` script has also been updated to use `dpkg-buildpackage`
