fix(frontend/forms): support nested array fields in forms

For example, when looking at the `users` array field in `Project`, previously it was possbile to access all properties
which are directly on the `user` object itself, by using `username` as the `fieldName` for example:
```json
const user: User = {
  username: ...
  email: ...
}
```

However, when a property was an object, it would not work, if for example `profile.status` is used as the `fieldName`:
```json
const user: User = {
  username: ...
  email: ...
  profile: {
    status: ...
  }
}
```

This commit changes that, to allow for using nested properties by using a `.` as a separator in the `fieldName`