Slash commands now block the composer while they execute.

### Steps to reproduce

1. Type a slash command which takes a while - e.g. /invite or /nick
2. Hit enter in the composer
3. Nothing appears to happen.
4. In practice, the command has started to execute in the background, but the composer no longer blanks out until the command has executed.

### Outcome

#### What did you expect?

On hitting enter, the composer should clear and the command should execute in the background.

#### What happened instead?

On hitting enter, there's no user feedback that the command is being executed at all (even though it is). And you can edit the command in the composer. And run it again. And generally cause chaos.

### Operating system

macOS

### Application version

today's Nightly

### How did you install the app?

_No response_

### Homeserver

matrix.org

### Will you send logs?

No