Use static modals for verification

Verification with cross-signing may requiring asking for your secret storage
passphrase, which is displayed in it's own modal on top of flows. For now while
verification takes in modals also, mark the verification ones as static so they
don't lose state when secret storage appears on top mid-flow.

src/components/views/messages/MKeyVerificationRequest.js

@@ -52,7 +52,7 @@ export default class MKeyVerificationRequest extends React.Component {
         const verifier = MatrixClientPeg.get().acceptVerificationDM(this.props.mxEvent, verificationMethods.SAS);
         Modal.createTrackedDialog('Incoming Verification', '', IncomingSasDialog, {
             verifier,
-        });
+        }, null, /* priority = */ false, /* static = */ true);
     };
 
     _onRejectClicked = () => {