* upstream/master:
  Fix medline tests...again (#2492)
  Make sure that unregistered event sources do not stop JabRef from shu… (#2487)
  Fix #2481: ClassCastException because of wrong cast (#2490)
  Catch NumberFormatException if context can't be parsed in groups (#2488)
  Improve CHANGELOG formatting
  Update guava from 20.0 to 21.0 and mockito-core from 2.5.5 to 2.6.2
  Fix aux duplicates (#2480)
  add update from DOI to the entryeditor sidebar (#2476)
  Remove obsolete import
  Add CHANGELOG entry (and one more link)
  Resolves #2309 JabRef freezes when importing unlinked PDF files into Database
  Update CHANGELOG.md
  Fixed bug when assigning refs to groups.
  emove html code from ACM fetcher before calling parser to prevent junk in bib file (#2473)