Add a system note when the issue/merge request description is edited

Description

Currently, editing an issue and a merge request is a completely silent operation that does not appear in the discussion flow of the issue/MR, therefore people coming back aren't notified that something moved unless they have memorized the previous content precisely and bother to process a diff in their head.

Proposal

• Add a system note when someone edits the description of the issue/merge request:
  • #24784 (closed) will add icons to system notes. For this system note, use a pencil icon.
• Complete the “Edited…” message under the issue/merge request description with the author of the changes:
• This event/system note does not send a notification email. (Users can leave a comment after editing the description if they feel it's necessary to send a notification to the subscribers.)