Improve Retraction Watch Database integration

[LukasRoeseler]

• Display of retracted studies could be clearer
• Decide whether to show expressions of concern, see also Add retractionwatch data to annotator #24
• Use the Retractionwatch Database API instead of the download link? API was published recently: https://www.crossref.org/documentation/retrieve-metadata/retraction-watch/

[LukasRoeseler]

Re API:

• Documentation: https://api.crossref.org/swagger-ui/index.html#/request-types
• Relevant code chunk:

  FReD/R/zzz_parameters.R

  Line 4 in 4896266

  #' However, you might want to use an older version, or even your own. For such advanced

[LukasWallrich]

Resolved in JS annotator - discuss if/how to do it in explorer