Check biblatex -> bibtex converter

[koppor]

if this was available, this would fix JabRef#1928 (comment)

(maybe it is already fixed. Need to search for test cases, ...)

[koppor]

Hint: There is "Cleanup entries", where one can use the "Convert to BibLaTeX format".

• There is no "Convert to BibTeX format" (in the dialog). We need to introduce that.
  • Introduce "journaltitle to journal" in the to-bibtex conversion
  • some more
• There is no documentation of "Convert to BibLaTeX format" (and the newly "Convert to BibTeX format"). It should be added to https://github.com/JabRef/help.jabref.org/blob/gh-pages/en/CleanupEntries.md.

Additonally:

• Remove "journal to journaltitle" conversion. Also remove it in the language string.
• We should fix the casing in the Screenshot. It should be "BibLaTeX" (see https://ctan.org/pkg/biblatex?lang=de).
• Fix Journal or Journaltitle jabref#4269

[koppor]

Fields mapping:
https://github.com/JabRef/jabref/blob/master/src/main/java/org/jabref/model/entry/EntryConverter.java

Tests:
(1) To BibLaTeX: https://github.com/JabRef/jabref/blob/master/src/test/java/org/jabref/logic/cleanup/ConvertToBiblatexCleanupTest.java
(2) To BibTeX: https://github.com/JabRef/jabref/blob/master/src/test/java/org/jabref/logic/cleanup/ConvertToBibtexCleanupTest.java