[FIX] mass_mailing: avoid breaking template layouts by stripping classes/data-attributes/style
[FIX] mass_mailing: avoid breaking template layouts by stripping classes/data-attributes/style

Rev. c9e38cae9f88bb61fd4468ad12e3e3200a4498ac made the html sanitizer
stricter, and it will not badly break any attempt at composing
a good-looking mail template.
It will also strip all data-* attributes due to the default
`strict` mode, so any theme/snippets used in the template
will be dropped/crippled after saving.

mass-mailing templates are strictly meant for composing
outbound messages, and never displayed directly in users' Inbox
or published on the website. So there does not seem to be any
strict necessity to sanitize them.

hip-odoo/odoo,dfang/odoo,ygol/odoo,hip-odoo/odoo,ygol/odoo,hip-odoo/odoo,hip-odoo/odoo,dfang/odoo,dfang/odoo,dfang/odoo,ygol/odoo,ygol/odoo,hip-odoo/odoo,dfang/odoo,ygol/odoo,ygol/odoo,hip-odoo/odoo,dfang/odoo,ygol/odoo