Purpose
=======

Improve the sponsors and attendees integration in event. Focus on :

  * sponsor management: transform it into a real business document allowing
    to communicate with sponsors) ;
  * attendee management: improve views, ease searching / updating them in
    batch;

Specifications
==============

Sponsors
--------

Move all the views related to sponsor and sponsor type to their own file to
clean module organization.

Add integration with mailgateway and activities through mail.thread and
mail.activity.mixin inheritance.

Improve sponsor views: add a real form view and add a small kanban view
displaying some partner / sponsor information.

Attendees
---------

In order to allow state management in batch, state field is made editable in
multi edit mode of registrations. That way event managers are able to work in
batch. It required to move some code currently called in custom action-called
methods directly in write. It makes sense to consider that some required code
is called by writing on state field instead of depending on calling the right
method. Indeed there is not much "business" code to move: calling mail
schedulers on confirmation, and putting the closing date when closing.

Event and registration tree views are improved, some fields are now editable
in multi edit mode.

When a user register on a event from website, it will create a sale order and
a partner (the customer of the SO). Set automatically the partner of the
registration to the partner of the sale order.

Other
-----

This merge include a fix in mass mailing (default domain propagation) and
web (void values display for phone widget). See sub commits for more details.

Task ID 2119333
PR #40949