Get log - time stamp ignored for manual (external) records

Having some manual (`source = "external"`) records in `log` table, the execution of 

```python
t = datetime.datetime.utcnow().strftime("%Y%m%d%H%M%S")
requests.get('https://localhost:5000/log?node_id=1&time=' + t, verify=False)
```

returns all records with time stamp **after** the given time `t` _but_ also all manual records with time stamp **before** the given time `t`.