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
The type is already enforced by the type annotation :-)

`@param int $level` will enforce that the type is an int or set the value to 0.

Btw. VERY cool that you use the AppFramework. Kudos for that!