[IMP] mass_mailing : improve interface, UI and usability

In order to improve and clarify the mass-mailing (Email Marketing) module
several changes have been made

1 - Clarify the name of the application:
2 - Prevent the “Quick Add/Create” feature in the mailing kanban view
3 - Label and wording clarification in mailing_mailing form view :
4 - Clarify button names "Send now" ("put_in_queue")
5 - Mailing Form view layout
6 - Clarify the required field
7 - Clarify the readonly field
8 - Wording clarification if scheduling mail in the future
9 - Add a Placeholders tools page in the form view:
10 - Clarify the Campaigns view
11 - Clarify the Action Helpers

TASK-ID : 2046078

closes odoo/odoo#36124

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>