Comments: add ability for site visitors to edit / delete their own comments

Nowadays, most social networks allow their users some control over the comments and posts they publish on the platform: you can often either delete your comments entirely, or edit them. This sometimes comes with some safeguards:

• When a comment / post has been edited, it is shown in the UI (sometimes with an edit date, sometimes with access to a revisions view)
• Edits are sometimes only allowed for a short period of time after a comment has been published.
• Platforms allowing you to delete your posts / comments will sometimes display a placeholder instead of a deleted comment, to avoid causing too much confusion for future readers by breaking the flow of the conversation.

I think that WordPress itself could potentially offer such a feature. We already do allow commenters to see their own comments while they're in the moderation queue. I think we could leverage that further to allow commenters to see an "Edit" and / or "Delete" link when they're visiting a post where they've commented in the past, as long as they're in the same browser.

We could offer additional options or controls:

• Site owners who want to retain full control over the conversations on their site should be able to turn off that functionality.
• The "Edit" / "Delete" links could be remove in theme templates.
• Site owners may want to limit this "edit" functionality to registered users on their site => this may actually be an easier implementation to begin with, where we would rely on the user being logged in to the site to show the "Edit" / "Delete" link.

For reference, here is a plugin that implements some of this functionality:
​https://wordpress.org/plugins/simple-comment-editing/