Folders - Remove content items from folder

First PR in what I assume will be a myriad of PRs. This one allows you to remove content items from a folder by issuing a DELETE request to `/api/folder/:folderId/library` with a `contentIds` body parameter that holds an array of content ids that should be removed
Can we also ensure that members, loggedin users, users from another tenant and anonymous users can no longer see the removed items?