emails: Reject sending emails with header parse errors.

Thanks to Jordan Milne (/u/largenocream) for
backing up our email queues^W^W^W^W^W bringing this to our attention. ;)
emails: Foil some basic header injections.

Thanks to "Jordan Milne (/u/largenocream) for this suggestion.
share emails: Prohibit sharing posts the user cannot view.

Thanks to a report by Jordan Milne (/u/largenocream).
Fix dead code references to controller "abort" method.

These are not the aborts you are looking for.