[IMP] mail: add activities allowing to define next actions on various models

This commit introduces generic activities to use in your addons. Activities are
actions user have to take on a document like making a phonecall or organizing
a meeting. Activities come with the mail module as they are integrated in the
Chatter but are not bundled with mail.thread.

New models are

 * mail.activity.type: used to categorize activities. Each type is a different
   kind of activity e.g. call, mail, meeting. An activity can be generic i.e.
   available for all models using activities; or specific to a model in which
   case res_model_id field should be used.
 * mail.activity: an actual activity to perform. Activities are linked to
   documents using res_id and res_model_id fields. Activities have a deadline
   that can be used in kanban view to display a status. Once done activities
   are unlinked and a message is posted. This message has a new activity_type_id
   field that indicates the activity linked to the message.

This commit introduces a mail.activity mixin to use in various addons that
enables the activities feature. It works like the mail.thread mixin. It defines
a activity_ids one2many field toward activities using res_id and res_model_id.
Various related / computed fields are also added to have a global status of
activities on documents.

Activities come with a new JS widget for the form view. It is integrated in the
Chatter widget although it is a separate widget. It displays activities linked
to the current record and allow to schedule, edit and mark done activities.
Use widget="mail_activity" on activity_ids field in form view to use it.

There is also a kanban widget defined. It defines a small widget to integrate
in kanban vignettes. It allow to manage activities directly from the kanban
view. Use widget="kanban_activity" on activitiy_ids field in kanban view to
use it.

Next commits will aim at integrating activities inside main Odoo addons.

Thanks to R&D India for their work and testing on this task. Thanks to belgian
Usabiliteam for reviewing and testing it. Thanks to @jem-odoo for the final
review. May his soul lie in peace with the trumpets of paradise.

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