Added load test to verify API, fixed the database service

.jupyter/api_authentication/__init__.py

.jupyter/api_authentication/api/user_endpoint_api.py

.jupyter/api_authentication/models/__init__.py

.jupyter/api_query/api/table_data_endpoint_api.py

.jupyter/api_table/__init__.py

.jupyter/api_table/models/__init__.py

.jupyter/load_test.py

.jupyter/test.ipynb

...rvice/src/main/java/at/tuwien/endpoints/UserEndpoint.java

...services/src/main/java/at/tuwien/service/UserService.java

...src/main/java/at/tuwien/service/impl/UserServiceImpl.java

.../src/main/java/at/tuwien/gateway/QueryServiceGateway.java

...in/java/at/tuwien/service/impl/IdentifierServiceImpl.java

...db/api/src/main/java/at/tuwien/api/user/UserBriefDto.java

...api/src/main/java/at/tuwien/api/user/UserThemeSetDto.java

...b/api/src/main/java/at/tuwien/api/user/UserUpdateDto.java

...ce/src/main/java/at/tuwien/endpoint/AbstractEndpoint.java

.../services/src/main/java/at/tuwien/mapper/StoreMapper.java

...rc/main/java/at/tuwien/service/impl/QueryServiceImpl.java

fda-ui/layouts/default.vue