Fix various Hibernate mapping oversights.

Change-Id: Iff22c68a718d6f65734e166cd08f4049ccc196ec
diff --git a/src/main/java/eu/mulk/mulkcms2/benki/newsletter/Newsletter.java b/src/main/java/eu/mulk/mulkcms2/benki/newsletter/Newsletter.java
index 7c9e50f..9c21e69 100644
--- a/src/main/java/eu/mulk/mulkcms2/benki/newsletter/Newsletter.java
+++ b/src/main/java/eu/mulk/mulkcms2/benki/newsletter/Newsletter.java
@@ -23,7 +23,7 @@
   @Column(name = "date", nullable = false)
   public OffsetDateTime date = OffsetDateTime.now();
 
-  @OneToMany(mappedBy = "owner", fetch = FetchType.LAZY)
+  @OneToMany(mappedBy = "newsletter", fetch = FetchType.LAZY)
   @OrderBy("date")
   public Collection<Post> posts;
 }