[file-audit] Show file download audit

Actually we can create a context manager to make this more reable. E.g.:

```python
@contextlib.contextmanager
def get_seafevents_session():
    try:
        yield SeafeventsSession()
    finally:
       session.close()

def get_file_audit_events(email, repo_id, start, limit):
     with get_seafevents_session() as session:
         events = seafevents.get_file_audit_events(ev_session, email, repo_id, start, limit)

```