Remove owncloud-sdk and re-implement it in web-client

[JammingBen]

The owncloud-sdk is quite old, does not fullfil our code standards and is hard to maintain because of the linking process. The latter makes future adjustments very time consuming, especially since we're planning to switch to id-based WebDAV requests.

Hence we figured it makes sense to remove the owncloud-sdk and implement its logic in our web-client package.

To-Do's

• Make sure Web uses the provided webdav abstraction instead of the sdk directly for webdav requests
• Port all WebDAV requests to a new abstraction in web-client
• Port OCS requests to a new abstraction in web-client (expect for the sharing stuff, which will be replace via sharing NG)
• Check if settings API needs to be ported -> no needed
• What about signUrl logic? -> refactor: move url signing to web-client package #10621
• Add proper (unit-) tests for the new code

[kulmann]

Hopefully pays off now that @dschmidt and myself already built an abstraction layer for webdav in web-client. 🙈