Refactor database initialization

[laurent-laporte-pro]

The solution is based on a refactoring of the init_db function and repositories. The aim is to modify this function so that the DBSessionMiddleware is no longer declared, as this middleware will be declared at the same time as the other middlewares.

Data initialization must be carried out in separate functions that will be called after database initialization. In this way, repository instantiation will no longer generate SQL queries.

On the other hand, some repositories will have to be modified so that a specific SqlAlchemy session is used instead of the global db variable. The db variable will be used if no session is supplied as an initialization parameter.

This feature is required to implement #1724