Implement folder download as archive

[labkode]

Story

As a user I want to download a folder (as an archive) without having to download individual components.
As a user I also want to select multiple files and download them as an archive.

Requirements

The archive is user-agent dependant (zip for windows and tar for Linux/Mac).

How to implement

This functionality has been implemented in the ownCloud 10 web by having a dedicated API:
https://github.com/cernbox/revaold/blob/master/ocproxy/api/api.go#L163

We can do better than that ugly URL.
However, the logic of the implementation is correct and can be re-used (files param, etc ...).

Another possibility is to use a GET request over WebDAV to a folder. This approach will open a can of worms. To avoid.

I'd propose to have:
/download_archive and respecting the same parameters.

We need input from front-end team:
@micbar

[labkode]

Ping @micbar