Don't allow null speakers on statements

[Betree]

• All new statements should be added with a speaker, and old statements without one should be migrated.
• Constraint should not be enforced in DB in a first time as we have existing statements with nil speakers.
• Frontend can be implemented first, independently of backend.

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:information_source: Some work was started on the frontend by @julien-leclercq here but PR was dropped after being staled for too long in a buggy state. Can be used as reference, but conflicts now prevent the code to be re-used.