VoIP: Fallback to matrix.org STUN server with a confirmation dialog

[manuroe]

We do not fallback anymore to the Google STUN server but users cannot make call anymore if their HS admin has not set up a STUN server.

We should fallback them to the STUN server available on matrix.org but with their permission.
We should do it when accepting or creating a call. If not possible, that will be another popup at app startup :/

The copy for the permission will come from riot-web: element-hq/element-web#10173 (comment) and next.

[manuroe]

Kamino cloned this issue to vector-im/riot-ios