Conversation System: Allow users to edit their comments

[vladolaru]

Option for post-submission editing of the comment, for a limited time (minutes). There is nothing more frustrating than noticing a typo or missing word after submission and having no way of correcting that. It feels that all the craft that went into your message was for nothing.

The Simple Comment Editing plugin does this, but I think we may have to integrate it for a more consistent experience (the same form fields as when posting initially).

This action should be AJAX-based.

Note: Related to #268