Enhancement: Download as archive

We've introduced archive downloads based on whether or not an archiver capability is present. The current implementation supports the archiver v2 (a.k.a. the REVA implementation).
Archive downloads are available in two different ways:
- as action on a folder (right-click context menu or actions panel in the right sidebar)
- as batch action for all selected files
The implementation is currently limited to authenticated contexts. A public links implementation will follow soon.

https://github.com/owncloud/web/pull/5832
https://github.com/owncloud/web/issues/3913
https://github.com/owncloud/web/issues/5809
