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
Given that this function does the Authz role updates, is `_removeContentItemsFromFolder` and this description the best way of describing what's going on? When reading this, I would expect this to be the function that removes the content items from the library index.