[IMP] mass mailing: improve demo data

Purpose of this commit is to have some demo data that are coherent. Main things
done in this commit are

 * clean the body of the first mass mailing given to the user in order to have
   a content more inlined with odoo emails;
 * update filter of the demo mass mailing to limit the results to something
   predictable, aka contacts of ReadyMat;
 * update statistics to be really linked to customer of the demo company by
   setting model and res_id;
 * create a statistics for each child of the company ReadyMat so that stat
   buttons redirect correctly to a filetered view;

This commit is linked to task ID 1885121 and closes PR #27003.

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