Refs #21852, #21850, #21851 - Speedup charts images creation

- Speedup savepngchart, setthumb and cleanup_thumbs methods by replacing
  ObjectModifiedEvent with InvalidateCacheEvent from eea.cache
[JENKINS] - Fix pylint warnings
Refs #21851 - Suppress sending useless emails while generating images
Merge branch 'master' of https://github.com/collective/eea.googlecharts