Add UI for saving a custom list

[rablador]

The user should be able to save a custom list to disk through a modal popup. Please note that the final design and text is not set yet, so there might be slight differences between current implementation and current design, as well as between current and final implementation.

You can test the new view by removing the exisiting code and pasting the following in TunnelViewController -> viewDidLoad() -> contentView.actionHandler -> .selectLocation. When you press the select location button you'll instead get the alert.

let viewModel = CustomListSaveAlertViewModel()
var alert = CustomListSaveAlert(viewModel: viewModel)

alert.didTapSave = { _ in
    viewModel.inputIsValid = Bool.random()
}

alert.didTapCancel = {
    self.dismiss(animated: true)
}

let vc = UIHostingController(rootView: alert)
vc.view.backgroundColor = .clear
vc.modalPresentationStyle = .overFullScreen
vc.modalTransitionStyle = .crossDissolve

present(vc, animated: true)

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This change is 

[linear]

IOS-509 Add UI for saving a custom list