Enable key backup after we reset it (#2170)

This ensures that we remember it if bootstrapCrossSigning gets called, so that
the authData gets signed by the master key, if a new key is created.

src/crypto/index.ts

@@ -953,6 +953,8 @@ export class Crypto extends EventEmitter {
             // sign with the device fingerprint
             await this.signObject(data.auth_data);
 
+            await this.backupManager.enableKeyBackup(data);
+
             builder.addSessionBackup(data);
         }