add linked projects and tasks variables in itil notification templates

[TarekRemo]

Checklist before requesting a review

Please delete options that are not relevant.

• I have read the CONTRIBUTING document.
• I have performed a self-review of my code.
• I have added tests that prove my fix is effective or that my feature works.
• This change requires a documentation update.

Description

• It fixes ticket 43722.
• Adds variables for linked projects and tasks in notification templates for ITIL notifications.
• List of variables added for linked projects :
  • [ItilObject].numberoflinkedprojects
  • linkedproject.id
  • linkedproject.url
  • linkedproject.name
  • linkedproject.content
  • linkedproject.code
  • linkedproject.planstartdate
  • linkedproject.planenddate
  • linkedproject.realstartdate
  • linkedproject.realenddate
• List of variables added for linked project tasks :
  • ticket.numberoflinkedprojecttasks
  • linkedprojecttask.id
  • linkedprojecttask.url
  • linkedprojecttask.name
  • linkedprojecttask.content
  • linkedprojecttask.planstartdate
  • linkedprojecttask.planenddate
  • linkedprojecttask.realstartdate
  • linkedprojecttask.realenddate

Screenshots (if appropriate):

[stonebuzz]

The code has been added to src/NotificationTargetTicket.php.
It will likely also need to be implemented in NotificationTargetChange and NotificationTargetProblem for consistency across notification targets.

[TarekRemo]

I think we don't have to add it in NotificationTargetChange and NotificationTargetProblem because we can't link project tasks to changes and problems but only to tickets.