Add system note on description change of issue/merge request

What does this MR do?

1. It adds a system note when an issuable's description has been changed:
   

2. It adds an author to the message Edited ... time ago by ... under an issuable's description:
   

Details

The current behavior of GitLab is this: any update of an issuable (title, description) or its relatated objects (labels, milestones, assignee, etc) does touch updated_at attribute of an issuable.
That means, if someone adds a label to an issue, everybody will see the issue updated.

This MR adds two more attributes: last_edited_by and last_edited_at.
These two are updated when title or description of an issuable are changed.
Any related objects (lables, milestones, etc) does not touch these attributes.

Ordering has not been changed by this MR.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests
  • Added for this feature/bug
  • All builds are passing
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #22162 (closed)