feat(NotificationMailling): add 'References' header for mail grouping (Gmail)

[stonebuzz]

From Gmail, mail grouping does not work.

To work GLPI notification need :

• Same subject ( 🆗 )
• Reference headers that reference IDs seen earlier in the thread, or have references headers that consistently refer to the same message ID ( ❌ )

https://workspaceupdates.googleblog.com/2019/03/threading-changes-in-gmail-conversation-view.html

this PR add custom header References with <GLPI-%uuid-%itemtype-%items_id>

Q	A
Bug fix?	no
New feature?	yes
BC breaks?	no
Deprecations?	no
Tests pass?	yes
Fixed tickets	!27170

[cedric-anne]

Waiting for customer validation.

Bug detected.

[cedric-anne]

I made some tests, and grouping notifications for a same item in Gmail requires additional changes. Indeed, mails are grouped only if subject is identical. I was able to achieve it on my GLPI instance by changing subject in Ticket notification template translation this way:

-##ticket.action## ##ticket.title##
+##ticket.title##

We should so probably consider changing default subjects of our notifications to make them identical across events. I guess such change could be made in another PR.

Anyway current PR will also prevent grouping of some notifications that are currently grouped. For instance:

• passwordexpires and passwordforget for a same User will not be grouped anymore, and that may be considered as an expected behaviour;
• plugin_formcreator_form_created, plugin_formcreator_need_validation, plugin_formcreator_accepted and plugin_formcreator_refused for a same PluginFormcreatorFormAnswer will not be grouped anymore, and it will probably be considered as a bug.

So, as it will change grouping behaviour on plugin notification, that could be considered as a BC-break, we should probably made such a change in main branch only, and produce a documentation to explain how to override the CommonDBTM::getMessageReferenceEvent() to customize grouping.

Also, we should review all core notifications to ensure that we are correctly define grouping rules where it is necessary.

I put it as a draft, as we need to decide what should and what should not be done to finalize this PR.

[cedric-anne]

As discussed IRL with @orthagh, we can merge this PR in 10.0/bugfixes branch.

Until now, all notifications related to the same object, depending on email client, were either always separated (in Gmail for instance), either always grouped (in Thunderbird for instance). With proposed changes, grouping instructions should premit to harmonize behaviour between email clients. This will change the way messages are grouped a bit, but it would not prevent end-users from receiving notifications, so the change is therefore acceptable in 10.0/bugfixes branch.

Note: As far as I understand, according to RFC2822, Gmail should group messages into a same thread even if subjects are different, but it is actually not the case. So, we still have to review default notification subjects, but it could be done in another PR.