More robust message editing

Here are four things that I think should be doable when you edit a message you've already entered:

1: Set the message to be a reply to another message.

2: Set the message to no longer be a reply to another message.

3: Set an image to retroactively be spoilered.

4: Set an image to retroactively not be spoilered.

I've certainly had issues with all four of these things in the past, and sometimes it's too long afterwards to delete the message and resend it completely.