fix: fix #1698 - Notification email doesn't contain Message-Id header

[schweigisito]

Description

This PR fixes #1698, an issue that notification emails don't contain Message-Id headers.

Related Issue

• Fixes Notification email doesn't contain Message-Id header #1698

Motivation and Context

This adds a quite important mail header which is needed to achieve

• tracking message threads and conversations
• detecting duplicate messages
• creating proper reply chains (along with In-Reply-To and References headers)
• debugging mail delivery issues
• proper spam detection systems

How Has This Been Tested?

Dev Docker image has been built and deployed using opencloud-eu/opencloud-compose

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Technical debt
• Tests only (no source changes)

Checklist:

• Code changes
• Unit tests added
• Acceptance tests added
• Documentation added

[rhafer]

I am not really sure if we need crypto save random numbers for the message ids. I guess a properly seeded pseudo random numbers from math/crypto could also be enough for that purpose.
But I guess it's fine this way as well.