Send email notification for changing milestone on issues and merge requests

Send an email notification for a changing milestone. This includes:

• Adding a milestone.
• Removing a milestone.
• Changing from one to another milestone.

Applies to issues and merge requests.

Notification level settings

• This should apply at the same level as changing assignees and labels. So that means this will be sent for the Participant level and Watch levels. See https://docs.gitlab.com/ee/workflow/notifications.html.
• For this issue, we will not add this to the custom notification settings.

Design

Email copy:

• Adding a milestone: Milestone changed to **11.5**
• Removing a milestone: Milestone removed
• Changing from one milestone to another milestone: Milestone changed to **11.5**
• The milestone name should be a link to the milestone page.