Ensure correct focus configuration for Element Call before allowing users to call.

[Half-Shot]

Presently anyone who enables Element Call via feature_group_calls can start an Element Call, but that is not enough to actually succeed. You need to have a foci configured on your client to do that, and so we should prevent users stumbling into this without first having foci configured.

Note, joining a call may be done by anothers focus presently so it's not required to have this config to join another call.