[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
And wouldn't it semantically make more sense to let the user manage the `locked`-state? For me the user is enabled just once, and can be locked/unlocked afterwards.