[IMP] mass_mailing; speed up kanban view

The kanban view displays mailing statistics for all mailings that are
sending or done. To compute these statistics, it uses the `total` as the
expected number of emails to send. This `total` used to be computed as
the number of records matching the mailing filter.
This is both slow (filters are seldom written in an optimized way), and
incorrect because the recipients were frozen when the email was sent.
The number of matching records may have changed since then.

To fix both problems, introduce a new `expected` computed field that
simply returns the number of emails that were put in queue at the time
of sending, and thus represent the real number of expected emails.
This field is computed along with the rest of the statistics.

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