Logfile download

implements / fixes https://github.com/owncloud/core/issues/12731

TODO:
- [x] add unit test
- [x] add warning if log file is bigger than 100MB
Can you please write an unit-test for that? - That can be done using reflection:

```php
$filename = \Test_Helper::invokePrivate($this->logController, 'getFilenameForDownload', [$timestamp]);
```