Migrate and uplink registration ID to bigint

[gregorbg]

The original idea was to "just" add a foreign key from RCE to registrations. Then I discovered two problems:

• registrations has a (very) non-standard ID type
• There are ~370K orphaned rows in RCE which need to be cleaned before the FK can be added

[gregorbg]

Sanity check query: SELECT * FROM registration_competition_events WHERE registration_id NOT IN (SELECT id FROM registrations);

[gregorbg]

Manually executed and registered on Prod