mail: Store mode flags by-value, not by-reference.

They are passed into the constructor by-value, and Clang warns that
we are taking the reference of a temporary.

Mail server: fix disabling notifications

* The default notifier didn't always take the setting into account.
* The mail server was not using the setting from the settings file and
instead waiting for a message that wasn't sent anywhere.

Fixes #10852.

Minor MailNotifier cleanup.

* Renamed to BMailNotifier, as it's part of the public API.
* Renamed Notifier.{cpp|h} to DefaultNotifier.{cpp|h} as that's the class it
implements.
* Made the mail counts uint32, and the byte counts uint64.

Mail server: fix disabling notifications

* The default notifier didn't always take the setting into account.
* The mail server was not using the setting from the settings file and
instead waiting for a message that wasn't sent anywhere.

Fixes #10852.

Minor MailNotifier cleanup.

* Renamed to BMailNotifier, as it's part of the public API.
* Renamed Notifier.{cpp|h} to DefaultNotifier.{cpp|h} as that's the class it
implements.
* Made the mail counts uint32, and the byte counts uint64.