Allow reactions to be retracted

[cellio]

A reaction is (in the user's opinion) true when it is added, but later changes (to the post or in the world) can cause it to no longer apply. We don't show when reactions were added (related: #703), so this can end up with people seeming to assert things they no longer consider true.

Yes we should show reaction times somewhere, but I'd like to also allow people to retract reactions. Ideally we would store this in the DB -- i.e. still record that a reaction was made and then retracted, as opposed to just deleting it. (I'd still like to get those reactions into post histories, plus this might be needed in rare cases for moderation.) If the reaction came with a comment I'm not proposing to delete that; I'm only talking about the reactions that show on the post.

[cellio]

I was thinking about this today because I was thinking about other ways for communities to handle possible duplicates short of my grand proposal, and realized that a "possible duplicate" reaction could be a path forward. This way there'd be a visible marker and an associated comment thread for discussion, yet the question would remain open.