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
When a content item's visibility is updated, don't we need to rebuild all of the user and group libraries it appears in as well? Can this all be done together?