Provide a replacement for WebDAV and FTP

[pbauer]

WebDAV and FTP are part of ZServer which is removed when running on Python 3. We might need a replacement if the uses-cases that we use it for are still valid nowadays.

@thet wrote in #2368:

Regarding

WebDAV and WSGI - there is a WSGI middleware for WebDAV (which also includes a standalone WebDAV server):

https://github.com/mar10/wsgidav
https://wsgidav.readthedocs.io/en/latest/reference_guide_architecture.html

This project looks quite promising:

It has a good software architecture - everything is implemented against interface specifications.
It comes with default DAVProvider, PropertyManager and LockManager implementations and allows for providing more specific ones.
It's tested against a lot of WebDAV clients: Windows File Explorer and drive mapping, MS Office, Ubuntu, Mac OS X, …

This could help us a lot providing WebDAV on WSGI for Plone/Zope.

[thet]

Plone 5.2.2 brings back WSGI, jipee!
https://community.plone.org/t/failed-to-be-enable-webdav-in-plone-5-2-1/12451/10