[database] Add migration for `users` table

This migration allows to either create or remove a `users` table from the database.
[database] Add migration for `user_id` foreign key
[composer] Require Doctrine DBAL as a dependency

Doctrine Database Abstraction Layer
https://github.com/doctrine/dbal

This is needed by the Illuminate schema builder to drop columns with SQLite.

This project has no explicit hardcoded requirement regarding RDBMS. The used driver depends on environment variables, thus allowing to use different ones in local and production scenarios.

We preemptively require this dependency so that everything goes smoothly in case one environment this projet has to run on depends on a SQLite databse.
Merge branch 'add-user-id-to-todo-items-table'