Persist user keys after fetch

lib/services/user_service.dart

@@ -4,6 +4,7 @@ import 'package:flutter/widgets.dart';
 import 'package:logging/logging.dart';
 import 'package:photos/core/configuration.dart';
 import 'package:photos/core/event_bus.dart';
+import 'package:photos/db/public_keys_db.dart';
 
 import 'package:photos/events/user_authenticated_event.dart';
 import 'package:photos/models/key_attributes.dart';
@@ -58,7 +59,9 @@ class UserService {
           },
         ),
       );
-      return response.data["publicKey"];
+      final publicKey =  response.data["publicKey"];
+      await PublicKeysDB.instance.setKey(email, publicKey);
+      return publicKey;
     } on DioError catch (e) {
       _logger.info(e);
       return null;