Developer reworks galleries and decouples them from Flickr tags

- [x] #19
- [x] create new `Gallery` model, to replace the cache as primary data provider and with unique attributes (`title, shorthand, is_portfolio` for now)
- [x] use new model decouple UI galleries from Flickr tags and use caches as data provider only (link galleries to flickr tags, use `Image` instead of `FlickrImage`)
- [x] refactor cache update API functionality to use the new models
- [x] refactor RESTful galleries functionality to use the new models
- [x] add new stuff to RESTful galleries functionality (update, destroy)
- [x] drag-and-drop reordering of galleries, analogous to gallery images
- [x] rethink, do we really need a local `FlickrTag` model at this point any more?