Configurable scores with exponential decay

Created by: njoyard

This PR implements exponential decay as shown in #63.

It also introduces a new Setting model to store administration preferences. For now they only contain score decay parameters ; default value are chosen so that score calculation stays the same as what we currently have.

There are no tests in the memopol_settings app because there is no business logic, only a model and an admin view. It is a separate app because it will most likely be used by the memopol app in the future (it has no reason to be in representatives_recommendations, but it can't be in memopol because then there would be a circular dependency between representatives_recommendations and memopol).