Enable chunking for bigger files in authenticated web upload

This commit adds chunked uploads in the Web UI (for authenticated users,
but not for public uploads). To do that the server endpoint used by the
uploader is changed from WebDAV v1 to WebDAV v2. The chunking itself is
done automatically by the jQuery-File-Upload plugin when the
"maxChunkSize" parameter is set; in "fileuploadchunksend" the request is
adjusted to adapt the behaviour of the plugin to the one expected by
"uploads/" in WebDAV v2.

The chunk size to be used by the Web UI can be set in the
"max_chunk_size" parameter of the Files app configuration. By default it
is set to 10MiB.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>