when a user sends a commit we instantly show our "best effort" pre-rendered draft before ajaxing in the right stuff

I think this is probably bad/undesirable, but I guess we can see how it works in practice.

In particular:

• We render invalid username mentions differently in previews vs submitted text.
• T4411 adds more similar rules.
• Users with a lost connection may believe their message sent when it did not.

I'm fine with the round trip, it's mostly that it looks like the entire thread is reloading that seemed overkill.