Download results in admin view

[zouharvi]

This PR adds a support to download results from the admin view and resolves #51.

• This PR is a rebased version of [campaign] update: admin to introduce an action to download the results #120. Thank you @seblemaguer, I put you as an author of the main commit.
• There were a minor tweaks needed to make it work. It looks as in the screenshot. If more than one campaigns are selected, then the output is a zip file.
• I didn't include writing the CSV headers because there are many functions that output CSV and should be part of a general system output overhaul (e.g. turn everything into JSON and output everything by default instead of per-task basis)

[zouharvi]

Accidentally forgot to rebase from #214, so please merge that first. 🙂