[openstreetmap/openstreetmap-website] Change subject of Note-notification to improve auto-grouping in email-clients (Issue #4047)

[Tobias]

### Problem

I use Gmail to read my mails. Usually it out-groups related mails.

Notifications about new comments and events on a note that I follow are not out grouped.

Gmail has some docs on this:
https://support.google.com/mail/answer/5900?hl=en
> Emails are grouped if each message meets the following:
> - The same recipients, senders, or subject as a previous message
> - A reference header with the same IDs as a previous message
>- Sent within one week of a previous message

Looking into the reference part I found https://flexport.engineering/sending-threaded-emails-using-rails-actionmailer-8549bd246283 which suggest adding a `reference` key to the `mail` method

---

Related to this is, that right now only the footer of the notification gives the information about the message that the update is related to ("More details about the note can be found at https://www.openstreetmap.org/note/3448122.").  Should the grouping not work, I would like to look into improving the subject or intro text to make it more clear which message the new comment/event belongs to.

### Description

All notifications on Notes should have the `reference` of the original note-creation, so all follow up mails are grouped by reference by email clients that support this kind of grouping.

### Screenshots

Those messages are ungrouped. It is not clear from the subject line which message they belong to.

<img width="1117" alt="image" src="https://github.com/openstreetmap/openstreetmap-website/assets/111561/5414022f-1df7-4973-8c2d-ea4ab1fed8b4">


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