Line comments

[amireh]

It would be very helpful if collaborators could comment on certain lines in an article. The tricky part here is maintaining conversations across revisions of lines. Here's a draft proposal:

• if a commented line gets modified, the conversation maintains a reference to the original version of the line on which the comment was made, and displays a notice that it may have gotten out of date
• if a commented line is removed, the author is asked to either move the conversation to a different line, or remove it
• if a conversation is "removed", it gets archived in some "Page Discussions" page