Some BibTeX tweaks

Not done with everything yet, but so far this addresses https://forums.zotero.org/discussion/37289/bibtexbiblatex-importexport-and-the-jr-name-part/

Still need to incorporate #745 and move some of this to BibLaTeX translator.

Thought I would give some heads up.
MSC is a classification and you can compare it with DDC (I have heard that every American knows the bibliographic classification DDC:) Besides the MSC classification we have also keywords e.g. https://zbmath.org/?q=an:06212000 . Mixing keywords with classification is getting a little messy, but maybe we will have such things anyway in a larger scale.