Message drafts should be transferred to upgraded rooms

### Steps to reproduce

1. Write a draft of a message in a room.
2. Upgrade the room.
3. See that the composer is now a "This room has been upgraded. Click here to continue the conversation" tile.
4. Click through to the new room and find an empty composer waiting for you.

### What did you expect?

To recover my message draft.

### What happened?

It seems unrecoverable unless you fish it out of LocalStorage and clean up the JSON-mangled text.

### Operating system

Arch Linux

### Application version

Element Nightly version: 2021092701, Olm version: 3.2.3

### How did you install the app?

The AUR

### Homeserver

_No response_

### Have you submitted a rageshake?

No