Button for download

[martinpopel]

In the Experiments overview window, there could be a button on each line for downloading the given experiment (source, reference and all the tasks [system translation], ideally with the config.neon files) in one zip file.

I think the easiest implementation would be to download it from the disk (not from the SQL database). Of course, if someone deletes the files from the disk (and leaves only the imported copy in SQL), this would not work, but that's OK for now.

Optionally, we could allow downloading only some tasks in the Tasks window: using the checkboxes the user can tick more than two systems, so we can just add a "Download selected tasks" button (it should probably include also the source and reference in the zip).