Show red shield for users that become unverified

For any users that we previously verified but that are not unverified, we will
now mark them and rooms they are in with a red shield.

Fixes https://github.com/vector-im/riot-web/issues/12808

src/utils/ShieldUtils.ts

@@ -5,6 +5,7 @@ interface Client {
     getUserId: () => string;
     checkUserTrust: (userId: string) => {
         isCrossSigningVerified: () => boolean
+        wasCrossSigningVerified: () => boolean
     };
     getStoredDevicesForUser: (userId: string) => Promise<[{ deviceId: string }]>;
     checkDeviceTrust: (userId: string, deviceId: string) => {
@@ -29,6 +30,13 @@ export async function shieldStatusForMembership(client: Client, room: Room): Pro
             verified : unverified).push(userId);
         });
 
+    /* Alarm if any unverified users were verified before. */
+    for (const userId of unverified) {
+        if (client.checkUserTrust(userId).wasCrossSigningVerified()) {
+            return "warning";
+        }
+    }
+
     /* Check all verified user devices. */
     /* Don't alarm if no other users are verified  */
     const includeUser = (verified.length > 0) &&    // Don't alarm for self in rooms where nobody else is verified