Make the email body take the entire space to avoid having some wasted space,
this will also make the user have more focus when designing an email.
Revamp the settings notebook page in order to give more clarity to the user,
and move some fields from the main form have been moved to this section to
have more space in the bottom for the email body.
In the mailing form, when the mailing is sent or is being sent, set fields
which are no longer useful for the user to change to readonly mode.
Add a wizard that enables the user to schedule a mailing, the schedule field is
still kept in the form for the user to be able to change the date when the
mailing is in the queue (if they want to send it sooner).
Display an action helper-style content when the email is empty, because,
currently, the user is left with a big white screen when the email has no
content which is not desirable.
Update the html_empty function to take into account style attributes to better
match the editor's void content.
Hide A/B testing fields from SMS mailing form view as these are not supported
for SMS marketing.
Task-2469409
closesodoo/odoo#68882
Ent-pr: https://github.com/odoo/odoo/pull/68882
Related: odoo/enterprise#18391
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>