[FIX] mass_mailing : renaming date GroupBy filters

Since modification on the group by filter on date field,
a forced value like group by date:month doesn't work anymore.
the user can choose the period but the forced value is not selected
when activating the group by.

Anyway, as it is more convenient to let the user choose the period
he wants to group by, the following groupby filters have been renamed
and the forced value have been removed.
In Mailings menu > Group By menu: rename 'Sent Month' by 'Sent Period'
In Mainling Lists > Group By menu: rename 'Creation Month' by 'Creation Period'
In Reporting > Group By menu: rename 'Scheduled Month' by 'Scheduled Perdiod'

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