Rework user management.

[mshima]

We currently have complicated logics to define when the User entity should be written and when user management should be written.
We can simplify it by using 2 variables:

• generateUserManagement to generate the user management code
• generateBuiltInUserEntity to generate the User entity and related implementations for user management and/or user relationships.
• generateBuiltInAuthorityEntity cassandra doesn't implements Authority entity.

---

Please make sure the below checklist is followed for Pull Requests.

• All continuous integration tests are green
• Tests are added where necessary
• The JDL part is updated if necessary
• jhipster-online is updated if necessary
• Documentation is added/updated where necessary
• Coding Rules & Commit Guidelines as per our CONTRIBUTING.md document are followed

When you are still working on the PR, consider converting it to Draft (below reviewers) and adding skip-ci label, you can still see CI build result at your branch.