feat: add guest account creation

### Additions

* added new mutation `generateGuests(count: Int!, expiration: DateTime!): [UserType!]!`

### Changes

* modified `UserType`, added fields `isGuest: Boolean!`, `expiration: DateTime`, `plainPassword: String`, `createdBy: RestrictedUser`
* modified `RestrictedUser`, added fields: `isGuest: Boolean!`, `expiration: DateTime`

Closes #428