Add `unlimited` as an option for max downloads when sharing a file

[schlagmichdoch]

This is probably linked to #21

I would love to have a possibility to share a file that only expires after a certain date but independently of the number of downloads.

To add this to the config we could either use the current expire_times_seconds like so:

expire_times_seconds: [0, 300, 3600, 86400, 604800]

or

expire_times_seconds: [-1, 300, 3600, 86400, 604800]

or we would need a new config boolean expire_times_include_unlimited .

I prefer the first option though