We still show the "add phone number" UX in user settings even if no identity server is in use

### Steps to reproduce

1. Disable your connection to the identity server
2. Try and add a phone number
3. Observe: a 500 error (_after_ you've been told that the SMS has been sent)

### Outcome

#### What did you expect?
- The add phone number UX should be suppressed when there is no identity server.
- Really, the "SMS has been sent" confirmation should be conditional on a 200 from the identity server send SMS request, not just included as part of the local echo.

#### What happened instead?
500

### Operating system

All of them

### Browser information

Firefox Nightly

### URL for webapp

app.element.io

### Application version

Latest

### Homeserver

matrix.org

### Will you send logs?

No