Renaming UsersModule, QuestionsModule, TopicsModule and GroupsModule

[Nictheboy]

I have to apologize for using names like UsersModule, QuestionsModule and TopicsModule which is not so proper. Maybe we should rename them into UserModule, QuestionModule and TopicModule, together with all the controllers' and services' class name. (GroupsModule is also under this naming convention to be consistent with these names.)

This is very easy to do thanks to VSCode's renaming functionality. However, it need to be done when all PRs are merged. This is the true difficulty :(

It's OK if we just don't fix this :)

[andylizf]

Glad to hear you remembered that the original groups.entity was renamed to groups.legacy sometime ago