[FEATURE] Lock users for authentication

Extend User Entity:

__Tasks:__

- [x] add new fields to User
- [x] update user authentication
- [x] add possibility to enable user in backend
- [x] update tests

__Informations:__

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| Tests pass?   | yes
| Fixed tickets | none
| Doc           | none
Isn't there always a value for `enabled`? I would just omit this check if there is no good reason. The password is IMHO an exception.