[REF] mail, note: move reminder feature from mail to note

This commit refactors f488dc58 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.