Add Ability to Delete Messages Locally (#70)

Merge branch 'develop' into f/delete-messages
Additionally added system commands to copy, delete or save attachments of selected messages

Reviewed-on: https://git.kske.dev/zdm/envoy/pulls/70
Reviewed-by: kske <kai@kske.dev>
Reviewed-by: DieGurke <maxi@kske.dev>

server/src/main/java/envoy/server/data/PersistenceManager.java

@@ -9,6 +9,8 @@ import envoy.data.User.UserStatus;
 import envoy.server.net.ConnectionManager;
 
 /**
+ * Contains operations used for persistence.
+ *
  * @author Leon Hofmeister
  * @author Maximilian Käfer
  * @since Envoy Server Standalone v0.1-alpha