We're reading the value from the database, but the
`ApplicationMailer.default` method is evaluated when the application is
started. So if we don't use a Proc, we'll need to restart the server
every time we change the value in the database, or else the old value
will still be used.
Using a Proc makes sure the mailer from address is evaluated at runtime,
so emails are sent using the from address currently defined in the
database.
The same situation took place using the devise mailer. Now we don't need
to check for the settings table being present because the Proc in the
devise initializer won't be evaluated before the settings table is
created and populated.
