[IMP] mass_mailing: update the snippet icons of the mail editor

On the right panel of the mail editor, the icons of the mass mailing
module do not match with the other icons. We will then update them to
improve the consistency of the layout. The icons will now have the same
background color as the other icons.

task-2658133

Co-Authored-By Carlos Valverde <cvs@odoo.com>

closes odoo/odoo#78163

Signed-off-by: awa-odoo <awa-odoo@users.noreply.github.com>