Reveal custom IS field only when required

This hides the identity server at first from the custom server auth flows. For
the flows that may need an IS if the HS requires it (registration, password
reset), we then check with the HS before proceeding further and reveal the IS
field if it is in fact needed.

Fixes https://github.com/vector-im/riot-web/issues/10553

src/components/structures/auth/Registration.js

@@ -499,6 +499,7 @@ module.exports = React.createClass({
                     serverConfig={this.props.serverConfig}
                     onServerConfigChange={this.props.onServerConfigChange}
                     delayTimeMs={250}
+                    showIdentityServerIfRequiredByHomeserver={true}
                     {...serverDetailsProps}
                 />;
                 break;