Use native TypeORM softDelete functionality

[michaelbromley]

In #21 I discuss the need for "soft delete" - marking an entity as "deleted" without physically removing it from the database. It is important to preserve the integrity of the data model among other things.

At that time, TypeORM did not have such functionality, so I implemented it in the application logic.

With the latest version of TypeORM, native soft delete is now available. The implementation seems to be very similar to my own, so switching should not be too difficult, and should simplify certain code paths and make things less error-prone.

[michaelbromley]

Note: PR in progess #304