Introduce more audits

.../edu/kit/scc/webreg/entity/audit/AuditApprovalEntity.java

...edu/kit/scc/webreg/entity/audit/AuditApprovalEntity_.java

...kit/scc/webreg/approval/ApproverRoleApprovalWorkflow.java

...c/main/java/edu/kit/scc/webreg/audit/ApprovalAuditor.java

...e/src/main/java/edu/kit/scc/webreg/audit/NullAuditor.java

...src/main/java/edu/kit/scc/webreg/event/AbstractEvent.java

...n/java/edu/kit/scc/webreg/event/ServiceRegisterEvent.java

...ice/src/main/java/edu/kit/scc/webreg/event/UserEvent.java

...ain/java/edu/kit/scc/webreg/service/impl/UserUpdater.java

.../java/edu/kit/scc/webreg/service/reg/ApprovalService.java

...a/edu/kit/scc/webreg/service/reg/RegisterUserService.java

.../kit/scc/webreg/service/reg/impl/ApprovalServiceImpl.java

.../scc/webreg/service/reg/impl/RegisterUserServiceImpl.java