Developer reworks galleries and decouples them from Flickr tags

- [x] #19
- [ ] create new `Gallery` model, to replace the cache as primary data provider and with unique attributes (`title, shorthand` for now)
- [ ] use new model decouple UI galleries from Flickr tags and use caches as data provider only
- [ ] migrate all galleries to use `Image` instead of `FlickrImage`
- [ ] cache updates update a gallery with tagged or individual images
- [ ] rethink, do we really need a local `FlickrTag` model at this point any more?