Look up references via database like CrossRef

[StevenClontz]

• DOI: https://www.crossref.org/education/retrieve-metadata/rest-api/
• MR: https://mathscinet.ams.org/batchmref ??
• math.SE and MO: https://api.stackexchange.com/docs/questions-by-ids
  • https://api.stackexchange.com/2.2/questions/3812324?order=desc&sort=activity&site=math.stackexchange
• Wikipedia: https://www.mediawiki.org/wiki/API:Main_page

[StevenClontz]

As noted in #64 while I was thinking this would be done client-side, it may make sense to instead include this in our build process and bundle this info in our JSON blob.