This commit refactors and improves the follow-up system to make reminder
handling more consistent and user-friendly.
1. Configuration flow updated
Follow-up levels were previously configured under:
Invoicing → Configuration → Follow-up Levels
They are now available under:
Invoicing → Settings → Automatic Invoice Reminder

2. Reminder timing improvement
Previously, users had to enter a negative delay to send reminders before the due
date.
A new field reminder_timing has been introduced to explicitly choose whether
the reminder is sent before or after the due date.
Delay values are now always positive.

3. Reminder logic refactored
Automatic follow-ups were previously based on the partner's follow-up level,
which could lead to inconsistencies.
Follow-ups are now based on the account move line follow-up level, ensuring more
accurate and consistent reminder progression.

4. Follow-up status removed
The `followup_status` field has been removed from `res.partner` as it is no
longer required.

5. Manual reminder from invoice
Users can now send invoice reminders directly from the invoice.
If `Send & Print` is executed again on an overdue invoice, the email template
automatically switches to the appropriate reminder template and attaches
the follow-up report.

6. Last reminder tracking
A new field last_reminder has been added to `res.partner` and `account.move`
This field stores the date of the last reminder sent for the customer or invoice.

7. New mail templates added
3 templates for manual reminders from invoice
2 templates for automatic reminders and batch reminders from invoice list view

task-5241209

com: https://github.com/odoo/odoo/pull/245403
upg: https://github.com/odoo/upgrade/pull/9587