[FIX] membership_extension: support restarted memberships

[yajo]

When a membership got cancelled and, later, restarted, we still had a cancellation date set, leading to the sensation that it was still cancelled.

With this refactor, when a cancellation date is earlier than a future start date, cancellation info is removed from the partner.

Before:

After:

BTW I improved a few things:

• Removed old dead code that is no longer necessary in v16.
• Fix a test that was checking what happened when a membership line was cancelled before its start date. It seems a wrong use case because memberships are only supposed to be cancelled during the membership, and not before starting it.
• Free members need to be considered members too.
• Migration script.

@moduon MT-8237