Introduce a proper database

[clemsos]

This will be useful if we are going to have more games because file system is a bit too minimal if we want to build a good web service.

The way to do it will be to clone the read/write in flat_files.py into a mongo.py or litsql.py and allow users to select using the config file. This will imply more changes (also from content.py) and may not be a priority but we can open this issue to discuss how to proceed to migrate the website to db while keeping flat file system as an option.