Developer decouples Images from (still linked) FlickrImages

- [x] new `Image` model that's optionally linked to a `FlickrImage` and also has sizes
- [x] `Image` working as a decorator for `FlickrImage` if the latter exists, for fields that aren't overridden
- [x] update of all existing functionality, no longer directly use `FlickrImage` anywhere
- [x] migrate all `BlogPost` instances