create pdf thumbnail previews (#15324)
add pdf preview image info to upgrade doc
code refactored / unit tests added
install ghostscript on travis ci
Merge branch 'feature/create-pdf-thumbnail-15324' into feature/folder-tree

* feature/create-pdf-thumbnail-15324:
  install ghostscript on travis ci
  code refactored / unit tests added
  add pdf preview image info to upgrade doc
  create pdf thumbnail previews (#15324)

Conflicts:
	Resources/config/services.yml
	UPGRADE.md