Add WebDAV COPY verb support


#1

@dmitrizagidulin wrote:

Add limited support for the WebDAV COPY method. (There is currently no easy way to transfer files from the client side from one URL to another.)

Enables:

ACL Interaction:

Copying from a non-LDP public Web resource to a container:
A COPY operation requires that the authenticated user has Write access to the destination container.

Copying FROM an LDP container to an LDP container:
A COPY operation requires that the authenticated user has Read access on the source container, and Write access on the destination container.

Interfacing with actual WebDAV servers:
Out of scope for the moment. We just need this as a convenience method to move to and from Solid servers.

There has been some discussion on https://github.com/solid/solid/issues/49 with @csarven and would like to invite more here.