R1:2e450212503e

Fix several issues with email-related global preferences

Summary:
Ref T11098. Mixture of issues here:

- Similar problem to D16112, where users with no settings at all could fail to fall back to the global defaults.
- I made `UserPreferencesQuery` responsible for building defaults instead to simplify this, since we have 4 or 5 callsites which need to do it and they aren't easily reducible.
- Handle cases where `metamta.one-mail-per-recipient` is off (and thus users can not have any custom…