Manage correctly a/b testing in mass_mailing.
We can now organize a mass_mailing campaign by testing
multiple mailings for the recipients targeted (subject,
templates, design, ...)
For this a new tab on the mailing record is added to better
promote the existence of the feature.
When an user has the group to manage mailing campaign, he can
access the tab A/B Test. This tab allows to enable A/B testing
for the mailing. If there is no campaign set for the mailing,
one is automatically created allowing the user to continue
smoothly.
Once A/B testing enable, the percentage of recipients use can be
set for each mailings. Also, the user can choose the deciding factor
that will set the final mailing as winner.
In a case, the user is not in manual mode, he can set the schedule datetime
for sending the final mailing.
task-2123242
COM PR: odoo/odoo#75622
ENT PR: odoo/enterprise#20454
UPG PR: odoo/upgrade#2781
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>