Use a new storage system for content (favorites, images)

[jbilcke]

Right now we use the local storage but it has limitations: about 5 Mb (some say 10 Mb)

which prevents us from storing images in it (such as the images deleted by OpenAI after generation)

One quick solution (until we can replace everything with a real API + database) is to use the IndexedDB:

https://www.raymondcamden.com/2015/04/17/indexeddb-and-limits