Use TMDb to fetch more info about movies#63

Initial idea: * Add a field to `Work` for `tmdb_id` * Add a "works" folder in `core.models.thumbnail_upload_path()` ? * Add `ThumbnailModelMixin` to `Work` * When saving a `Work` in the Django Admin, if either `imdb_id` or `tmbd_id` is set then use [the TMDb API](https://www.themoviedb.org/documentation/api) to fetch the other ID (if missing) and fetch the poster image and save it. * Add a management command to run through all `Works` of kind "movie" and populate any missing IDs or posters. * On a `Work`'s detail page, display its thumbnail, if any, and link to its TMDb page if it has a `tmdb_id`. In future we could fetch further data about a movie but these are the main ones I'm after now.