Current activity types master data cannot be unlinked through a quite harsh
override of unlink using xml id existence, introduced at 2602642a78dff6c15d615fadf4c55a6b6ca315df.
Some of them are real master data, other could be unlinked if users want it.
Finally custom activity types may cause errors when removed if used in some
side models like server actions or automated actions.

To allow more flexibility we remove the removal constraint and implement
a fallback mechanism. It is implemented in mail.activity.mixin when calling
activity_schedule(xmlid, ...). If the given xmlid is not found
_default_activity_type() specifies the default activity to use. It can
be overridden in modules if some specific behavior is wanted.

e.g.

def _default_activity_type(self):
    """Define a default fallback activity type when xml id not found
    only used in in activity_schedule() for now.
    """
    try:
        return self.env.ref('mail.mail_activity_data_todo')
    except Exception:
        return False

In this commit we also

  * restrict deletion of activity type (hr.plan and ir.actions.server);
  * fallback on default activity type if xmlid ref not found in
    activity_schedule;
  * correct calls to activity_schedule and replace some manual creation calls
    by activity_schedule;

Task ID 1961156
PR #39013

Related: odoo/enterprise#6238
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>
Co-authored-by: Victor Feyens <vfe@odoo.com>
Co-authored-by: Michaël Mattiello <mcm@odoo.com>

This commit refactors f488dc58c37fdc7fd49df759165de641e213444b that added reminder feature in activities.
After merging it was suggested specifications were not correct and it has
been decided to move the whole reminder feature in note.

Reminders are now notes instead of activities. Now when having note appµ
systray proposes to create a note / reminder. A note.note record is created
with info given by the user. If a date is given in the systray quick create
then an activity is scheduled on this note.

This means user can create records in activity systray that are not
activities but notes used as reminders. Those will use the first availableµ
note column for the user.

As reminders are now notes the support of activities not linked to any
document is removed, as well as archiving activities and tests linked to
that behavior.

This commit has been not carefully tested nor in-depth reviewied. It is
considered as too important for the upcoming freeze. This closes task ID
1833628. Closes #24129 . Many thanks to @dpr-odoo for its quick development
and good work on this task.