bibtex consistency check

[dkg]

Before a site build, there should probably be a bibtex consistency check run, and the build should abort if the check is not clean.

things that could go into such a consistency check:

• ensure that the file is valid bibtex
• ensure that tags are well-formed (see tags that contain a # symbol don't work right #45)
• ensure that article identifiers are unique and not overwritten (see entry URLs will change if the entry changes or if the code for generating the hash is non-reproducible #34)
• ensure that text is in some normalized form (see Normalize data.bib to Unicode NFKC #32)

If there's a failure, the consistency check should be verbose about the problem and indicate what kinds of fixes are necessary.