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
We also need a test that verifies that everything is working as expected when we try to remove a mix of existing and non-existing content ids from a folder