[IMP] mass_mailing: simplify read_group column expansion
[IMP] mass_mailing: simplify read_group column expansion

The framework supports a special `group_expand` attribute on fields,
to provide a helper method that expands the list of columns in a
read_group (typically meant to display empty columns in kanban views)
See rev. 580e78ad375aa3f52f2307c82281268796275aea

More efficient, and less code.

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