[14.0][ADD] mail_routing_subject

[hbrunn]

This module matches incoming mails by subject to existing mails, thereby
easing the problems caused by broken mail clients that don't include the
original mail's message id in a reply, which standard Odoo uses to match
emails.

Configuration

To configure this module, you need to:

1. Go to Settings / General Settings
2. In the Discuss section, fill in all prefixes that mark a subject as a
   reply, such as Re: RE: Aw: AW: in the field Reply prefixes

Usage

To use this module, you need to:

1. Write a new mail with a subject that makes it look like a reply to
   an existing mail sent to you by your Odoo installation by prefixing
   one of the reply prefixes you configured
2. Send the mail, have Odoo fetch it
3. Observe that the mail will be attatched to the thread your existing
   mail is attached to