Track milestone changes explicitly for system notes

• Track milestone changes explicitly in the system, as events.
• System notes should use this new data architecture.
• Applies to project milestones and group milestones.
• Applies to issues and merge requests.
• In each event, track the following:
  • Who made the change.
  • What the previous milestone was. Could be none.
  • What the new milestone is. Could be none.
  • The timestamp.

System note text copy:

• @victorwu changed milestone to 11.1 · less than a minute ago

• @victorwu removed milestone · less than a minute ago

• The milestone should link to the milestone page if it exists.

• If the milestone does not exist (it was deleted), render the milestone title as regular text without a link.

• This text should be internationalized to support translations.

• We will not migrate any old system note data. These requirements only apply to new changes to milestone change events, and how system notes are presented.