Create a way to parametrize the context of a website

[Skylsmoi]

We will need a way to define different values depending on the context.

For example, linking to preprod-my.domain.net or my.domain.net depending on the environment.
A possible solution is to:

1. create jssg/settings.py.sample added in git with every configuration parameter and their default values
2. add documentation to create the jssg/settings.py from jssg/settings.py.sample when installing the repository
3. add jssg/settings.py in .gitignore (which doesn't exists yet)