Support foreign keys with SQLite

[kevinpapst]

Fixes #589

There is now a Doctrine EventSubscriber that listens on the postConnect event and triggers a

PRAGMA foreign_keys = ON;

in case that a SQLite connection is found.

This finally activates foreign keys and cascade deletes with SQLite!

See https://www.sqlite.org/pragma.html#pragma_foreign_keys

[lock]

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