[FIX] event: prefer registration email over partner email

As of rev. ca989b6548e93c25f54684e5569ed3998161acce, the default
recipients of a model record are used to filter out blacklisted and
duplicate emails during a mass-mailing. This can match both either
partner_ids or emails, as returned by
`message_get_default_recipients()`.

The problem with even registration is that a single partner, sometime
anonymous (Public User) can be used to register several participants.
We therefore need to override `message_get_default_recipients()` to that
intent, similarly to what is done in crm.lead since
ca989b6548e93c25f54684e5569ed3998161acce.

Without this, mass-mailings sent to free event attendees would never be
sent, as they would all appear to be duplicates of each other.

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