Replaces the yes/no notify_email flag with notify_mode:
- off: no mail
- owner: only the board owner, as before (migrated from notify_email=1)
- board: everyone with access to the board, filtered by their own Deck
"Notifications" setting for it (board:<id>:notify-due: off /
assigned / all), mirroring Deck's NotificationHelper rules
Recipients are resolved via PermissionService::findUsers() (owner, ACL,
groups and circles expanded), AssignmentMapper::isUserAssigned() and
IConfig, none of which reads Deck's frozen $userId. If they cannot be
resolved, no mail is sent rather than one to the wrong people.
Moved cards are collected over the whole run and each recipient gets a
single digest mail afterwards, in their own language.
Bumps to 35.2.0 so the migrations run.
