Proposal: Ability to add comment on messages/trees

[notmd]

Add the ability to add comments on messages/trees. Use case:

• A note that can help the next users (mods) reduce the review time signification of the message. This will be the primary usecase. Most of the time I skip a task because I'm too lazy to search for the fact of the message. Having a link to somewhere will make me reduce my skip rate (possibly reduce halted_by_moderator rate).
• Users can see others' opinions about the message.
• Users can leverage this to report grammar errors. So mods (or new role editor) can apply the change, similar to the GitHub review process.
• Ability to force users who create "prompt reply pair" to add a source for the Q/A.

We can leverage/combine the flag_message for this feature. Also, resolve #1155

[andreaskoepf]

(I am not against it but comments will require full moderation, support, delete etc.)