Bugfix: Check names also for folders or files that currently are not visible

We've changed the way how web checks if a file or folder exists.
From now on it also include files from the current folder that actually are not visible.

This was problematic in situations like the pagination, where a file or folder was not
available in the current set of resources and the user tried to create a folder with the same name.

https://github.com/owncloud/web/pull/5583
