Lock issue and merge request

Having a discussion lock mechanism would be really helpful for 2 major reasons:

Discussions that are several years old and the issue/MR is closed, but people continue to try to resurrect the discussion.
Discussions where someone or a group of people are trolling, abusive, or in-general are causing the discussion to be unproductive.

This feature applies to both issues and merge requests.
You will be able to lock an issue or lock a merge request.
The resulting mechanism is the same (i.e. locking down the discussion thread.)
The designs below are shown for issues. They implementation should be analogous for both issues and merge requests.
For a locked merge request, the comment thread is locked. And the diff discussions are also locked. So you are unable to add comments to any existing discussion thread in a code diff (or start a new one).

Design

Nonconfidential + Unlocked	Nonconfidential + Locked	Confidential + Unlocked	Confidential + Locked

Lock issue	Unlock issue

The Edit link is only visible to users who have permissions to toggle the lock. These are project members only.
System notes:
- Victor Wu @victorwu locked this issue 2 hours ago.
- Victor Wu @victorwu unlocked this issue 1 hour ago.
For system notes, use the associated icon.

Nonconfidential + Unlocked	Nonconfidential + Locked	Confidential + Unlocked	Confidential + Locked

Nonconfidential + Unlocked	Nonconfidential + Locked	Confidential + Unlocked	Confidential + Locked

It doesn't have any icons before the text because 2 icons + text might be too much.
It doesn't have [Learn more] at the end of the text. Instead, users will go to the documents by clicking confidential and locked directly.

Locked: This issue is locked. Only **project members** can comment. [Learn more]
Locked + Confidential: This issue is [confidential] and [locked]. People without permission will never get a notification and not be able to comment.