Create cross-signing keys during authentication

With this change, Element now creates cross-signing keys during auth flows for
password login. For other auth flows like token / SSO, it will not happen until
a cross-signing / secret storage dialog flow as before.

src/components/structures/MatrixChat.tsx

@@ -1876,6 +1876,13 @@ export default class MatrixChat extends React.PureComponent<IProps, IState> {
         return this.props.makeRegistrationUrl(params);
     };
 
+    /**
+     * After registration or login, we run various post-auth steps before entering the app
+     * proper, such setting up cross-signing or verifying the new session.
+     *
+     * Note: SSO users (and any others using token login) currently do not pass through
+     * this, as they instead jump straight into the app after `attemptTokenLogin`.
+     */
     onUserCompletedLoginFlow = async (credentials: object, password: string) => {
         this.accountPassword = password;
         // self-destruct the password after 5mins