Handle the case of no IS in features that require IS to lookup

[jryans]

Cloned from element-hq/element-web#10619.

Since it's now possible to be disconnected from an identity server entirely (your session has no active identity server), it's unclear how features that require an IS to function should behave.

For the case of the Discovery section in Settings, it's easy enough: you have no IS, so we don't show any 3PIDs to control.

However, there are other IS-using features in Riot, like:

1. Inviting by 3PID
2. (There might be more such features in mobile, so please add if so.)