* [8.0][mass_mailing] some fixes
* [FIX] rename wrong mailing_type field and add missing fields when creating a mass mailing from the composer
* [FIX] coerce tooltip to an unicode string to avoid a json crash when locale produces a non unicode string for strftime(%B)
* [FIX] repair wrong sql statement computing statistics
Merge pull request #1776 from acsone/8.0-jne-mass_mailing-improvements

[FIX] Mass mailing: fixed statistics computation + summary graph tooltips (using ustr) + values of the mass mailing when creating it from the mail composer wizard. Courtesy of Acsone.