The TODO file is, currently, empty, so remove it.

The only entry in here was addressed a few months back.
Move comment to the proper place.
Make a few comments better reflect reality.

No functional change.
Fix crash if a message is incorrectly edited by a user.

One could argue that people get what they deserve in such an instance, but I
managed to trigger this crash, so I'd like to at least save myself this
particular ignominy in the future.