[ADD] mass_mailing_slides: allow to mass mail course attendees

PURPOSE

eLearning should have its own application in manager. Indeed this is becoming
a big application and having it embedded inside Website application is not
enough anymore. It should also hold updated and easy-to-use menus,
navigation, actions and views in backend.

SPECIFICATIONS

Integrate a button on slide.channel form view allowing to perform a mass
mailing on course attendees. Do a flow similar to event.

Add menu entry in menu of slide.channel kanban card after invite
  * Mail Attendees (same as on the course card and depend of the setting)

Add menu entry inside Settings
  * Settings: add an option to install module mass_mailing_slide

LINKS

Task 1978729
PR #35061

Co-Authored-By: Thibault Delavallée <tde@odoo.com>
Co-Authored-By: Jérémy Hennecart <jeh@odoo.com>