Implement 3rd-Party Rules

[hohwille]

Implement the 3rd-Party-Rules with ArchUnit.

Acceptance-Criteria:
The following rules are implemented with ArchUnit:

• E1 - @transactional
• E2 - Mysema (vs. querydsl)
• E3 - proper JPA usage
• E4 - proper Hibernate usage
• E5 - guava Objects (vs. java.util.Objects)
• E6 - @org.hibernate.Type or @javax.persistence.Convert

[hohwille]

Rules are implemented but some rework was left in PR #22 comments (optional scope) that can now be addressed with a new PR to complete this story.