[MERGE][REF] mail, various: move email notification templates from jinja to QWeb

This merge changes the templates used to send notification emails. They
are currently defined as mail.template using jinja. We choose to make
them QWeb views. There are several reasons to do so

 * those are not real mail.template records. Indeed they cannot be used
   outside of the notification process as some values are computed
   and not available on the mail.message record used to render the
   template;
 * we do not really need other fields than body. Indeed fields like
   subject, email_from or email_to are computed from the notification
   process;
 * we do not want people to update the mail.template without knowing
   the consequences, especially for fields like recipients that may
   broke the mail gateway;
 * using the html editor easily break the mail.template as it is very
   custom and very to break without really realizing it.

It also simplifies template management as it lessens number of mail
templates people have in their list view of mail.template. It avoids
mixing technical and functional templates.

We move to QWeb templates as those are not too hard to customize and
allow to perform body rendering which is what we really need when
notifying people of a new message.

This merge also factorize the "custom layout" used in several addons
to use a single secondary template defined directly in mail. Indeed they
are using the same template. Therefore having it defined only once helps
editing or fixing it.

Please see sub commits for more details about the merge.

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