DDBNEXT-614: implement newsletter service.

  The newsletter service communicate with Elastic search for storing
  and retrieving information related to newsletter subscription.
Merge branch 'develop' of https://github.com/Deutsche-Digitale-Bibliothek/ddb-next into develop
DDBNEXT-641: add integration test for newsletter service.
Merge branch 'develop' of https://github.com/Deutsche-Digitale-Bibliothek/ddb-next into develop
Merge branch 'feature/DDBNEXT-641' into develop