feat: change users query and change user data to use group enum

### Changes

* `FilterUsersInput` field `group` renamed to `groups`
* `FilterUserInput` field `groups` type changed to a list `GroupEnum`s
* `ChangeUserInput` field `group` type changed to `GroupEnum`

### Deletions

* remove `isStaff` and `isSuperuser` from `UserType`

Closes #249