Fix configuration for eslint (linting subdirectories; activating jest rules

[BePo65]

The configuration for eslint had 2 minor topics:

• lintingwas only activated for directories 'src', 'test' and 'build' but not for the subdirectories of these directories
• jest rules for eslint were loaded (property "plugins" in .eslintrc.json) but not activated

Fixes these topics and removed the newly identified warnings.

[codecov[bot]]

Codecov Report

Merging #1926 (7efe3cc) into dev (9ef1714) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##               dev     #1926    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files          175       181     +6     
  Lines         1697      2064   +367     
  Branches       387       538   +151     
==========================================
+ Hits          1697      2064   +367     

Impacted Files	Coverage Δ
src/locale/bo.js	100.00% <ø> (ø)
src/locale/br.js	100.00% <ø> (ø)
src/locale/de.js	100.00% <ø> (ø)
src/locale/es-do.js	100.00% <ø> (ø)
src/locale/es-pr.js	100.00% <ø> (ø)
src/locale/es-us.js	100.00% <ø> (ø)
src/locale/es.js	100.00% <ø> (ø)
src/locale/fi.js	100.00% <ø> (ø)
src/locale/it-ch.js	100.00% <ø> (ø)
src/locale/lt.js	100.00% <ø> (ø)
... and 95 more

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