The original implementation of this migration had a highly unfortunate
bug that would result in it deleting all reactions to realm emoji on
the server; we missed this in review, so essentially all historical
realm emoji reactions on chat.zulip.org were lost :(.
We both correct the problem, and also add logging of the deleted rows
that would help should anything be deleted erroneously.
If an emoji that was deleted was the only realm emoji, or more
generally if all realm emoji were deleted, then we would just leave
the reaction unchanged, with an `emoji_code` that is now corrupt.
Instead, treat this case the same as if only this emoji was deleted
while others remain.
Till now, we had been storing realm emoji's name in emoji code field
in reactions' model. This commit migrates it to store realm emoji's id.
It is a part of effort to migrate realm emojis to be referenced by their
id and not by name.
