Rewrite nextcloud plugin

Summary: Let CopyJob and the webdav KIO slave do the hard work. Run the GetCredentialsJob async. Use generic dav-storage service type instead of nextcloud-upload (introduced in D21738).

Test Plan: Shared one or multiple files to my nextcloud

Reviewers: Frameworks, apol, bshah

Reviewed By: apol

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D27662