Editing message topic only moves one message

[WesleyAC]

When you edit a message topic in mobile, it only moves the one message. On web, we give three options:

• Change later messages to this topic
• Change only this message topic
• Change previous and following messages to this topic

We should offer these options. Probably the best UI for this is to just pop up a modal when the user clicks the checkmark button to submit the edit, since I think it's likely to be too crowded elsewhere. (That'll also make sure that the edit of the topic wasn't inadvertent)

[gnprice]

I agree with that plan about the UI.

Adding this feature should probably block on fixing #2688 (and in particular its symptom #4830.) That issue makes the experience of editing the message topic feel pretty broken -- if you're looking in a topic narrow, the message doesn't actually disappear from the message list -- and so I feel like making it easier / more natural to go editing topics would increase the annoyance there.

[chrisbobbe]

#2688 has been fixed, so I'll mark this as unblocked.