We have no UI indication when the app is stuck syncing and showing stale room history.

### Steps to reproduce

1. Where are you starting? What can you see?

I launched app.element.io for the first time since August

### Outcome

#### What did you expect?

I expected to see a spinner at the bottom of the timeline to let me know that EW is blocked doing a massive gappy sync, and that the history it's showing me is stale.

#### What happened instead?

I have no way to tell that the history is stale; it looks like i'm viewing a normal instance of the room, and i have no way to tell that the client is trying to sync.

A good enough solution would perhaps be to:
1. Show a spinner at the bottom of timelines the first time you call /sync after launching the app
2. If /sync takes significantly more than 30s to return, show a spinner at the bottom of timelines, or some other UX idiom to show that in practice the client is currently failing to sync from the server.

### Operating system

macOS

### Browser information

Chrome

### URL for webapp

app.element.io

### Homeserver

matrix.org

### Will you send logs?

No

Reproduce the stale-history state in app.element.io after a long gap, using the issue's macOS, Chrome, and matrix.org context. Trace the first /sync after launch and the timeline UI; done means users can distinguish an in-progress or stalled sync from normal room history, with the chosen indication covered by an appropriate test.

Written by the indexing model from the issue text.