[MERGE][REF] mail, various: make templates great and bioutifoul again

In this commit we improve templates used in nearly all odoo addons. Purpose
of this commit is to have templates that embed or use standard Odoo email
layouts to make them look modern and have a common style across all emails.

Main guidelines

 * better use of div / p / br to try to lessen layout issues, especially
   when updating templates using the editor;
 * correctly sequence the templates fields definition;
 * correctly set templates values notably auto_delete and user_signature
   fields to avoid confusion;
 * correctly layout the email content using light notification email. It
   can either propagate the layout choice through various send mail methods
   or directly embed the styling in the templates for more technical or
   complex templates;
 * use email_formatted computed field when possible to avoid having hand-made
   from / to addresses;
 * fix various typos and improve subjects when necessary;

Content of emails is not necessarily updated as the purpose of this task is
about styling, not content itself.

This merge also contains some additional code allowing to layout email sent
through the send_mail of mail.template and the send_mail of survey.compose.
message. It allows to strip the layouting part of the email and keep only the
content in the template body. Some technical templates keep the full layout
to keep the possibility of easily send them through various pieces of code.

This merge is linked to main task 1868112 and closes main PR #25889. It is the
summary and review of several tasks and several PRs:

  * task ID 1843361 and closes PR #25287
  * task ID 1843171 and closes PR #25293
  * task ID 1843395 and closes PR #25294
  * task ID 1843122 and closes PR #25295
  * task ID 1843360 and closes PR #25299
  * task ID 1843224 and closes PR #25313
  * task ID 1843151 and closes PR #25346
  * task ID 1843376 and closes PR #25349
  * task ID 1841503 and closes PR #25388
  * task ID 1843136 and closes PR #25412

Thanks to all contributors and testers for this task !