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 settings) more explicitly.
• When metamta.one-mail-per-recipient is off, remove the "Email Format" panel for users only -- administrators can still access it in global preferences.

• Deleted a user's preferences, changed globals, purged cache, made sure defaults reflected global defaults.
• Changed global mail tags, sent mail to the user, verified it was dropped in accordinace with global settings.
• Changed user's settings to get the mail instead, verified mail was sent.
• Toggled user's Re / Vary settings, verified mail subject lines reflected user settings.
• Disabled metamta.one-mail-per-recipient, verified user "Email Format" panel vanished.
• Edited "Email Format" in single-mail-mode in global prefs as an administrator.
• Sent more mail, verified mail respected new global settings.