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
It's based on a similarly named function `_addContentItemsToFolder`. Although I can see where the confusion comes from, I don't think it's necessarily incorrect as a folder is just a special authz group with an extra library for content items.