SSL: error on unknown CA certificate (e.g., self-signed).

When entering the HTTPS URL of a homeserver that uses an SSL certificate signed by an unknown CA (e.g., where the CA has a self-signed certificate and is, therefore, unknown), RiotX displays an error (in a toast popup) and will not permit login. :disappointed:

Old-riot used to ask you confirm that you wanted to use the server and remember the signature.