[JSTOR] Modernize plus a few tweaks.
* General code update to make it easier to follow
* Remove betaSearch support (discontinued by JSTOR)
* Validate DOIs using doiRA tool, which would maintain DOIs registered with non-CrossRef service and is quite lightweight
* Test updates are mostly just rearrangements. A few DOIs got changed by JSTOR.
* Add support for /info URLs re https://forums.zotero.org/discussion/43056/saving-jstor-items/ (as displayed after following a DOI)
* Remove `/\/i\d/` test for search results, since it seems very general and we don't have a test case