As mail grows and will continue to grow, ordering imports and having right
files naming as well as right content in them is important to understand
module organization and content.
Containing
* reorder init file to give an overview of models;
* move ResGroups override in its own file, outside of res_users file;
* split file containing MailActivity model and MailActivityMixin so that
mixin is contained in its own file, easing followup;
* split Activity and ActivityType models into two files;
No functional change comes with this commit. This is only code move.
Task-2631873
PR odoo/odoo#75571
Thibault Delavallée