Enforce Secure Backup completion when requested by HS

This removes all buttons to escape the Secure Backup setup flow when the
matching `.well-known` setting is set by homeserver.

Part of https://github.com/vector-im/element-web/issues/14954

src/utils/WellKnownUtils.ts

@@ -33,3 +33,8 @@ export function getE2EEWellKnown(): IE2EEWellKnown {
     }
     return null;
 }
+
+export function isSecureBackupRequired(): boolean {
+    const wellKnown = getE2EEWellKnown();
+    return wellKnown && wellKnown["secure_backup_required"] === true;
+}