Should be able to queue a thread reply while thread root is sending/encrypting

### Your use case

#### What would you like to do?

In a room with lots of devices (taking a long time to encrypt), I'd like to be able to send a root message and threaded messages so everything lands in the room nicely, as is possible with edits.

#### Why would you like to do it?

Currently I have to wait until encryption is complete, meaning I have to hold onto my thoughts for minutes at a time instead of quickly getting them into the room to encrypt over time.

#### How would you like to achieve it?

Just like edits: show the thread button normally, and make the events valid once proper event IDs are known.

### Have you considered any alternatives?

_No response_

### Additional context

_No response_