* Refactoring: Move AppItem to Client lib and use the class for item list
This makes it available for the app, which would otherwise have to replicate the model. Also uses the proper class for the item/perk list of the app models.
* Remove unused app item payment methods property
* Do not ignore nullable values in JSON
* Revert to use Newtonsoft types
Items with type topup have a price = null and hence not even the property set (ignored in JSON). This needs to be handled in the temlate, otherwise this exception occurs:
```
An unhandled exception was thrown by the application.
System.InvalidOperationException: Nullable object must have a value.
at AspNetCoreGeneratedDocument.Views_Shared_Crowdfund_Public_ContributeForm.<>c__DisplayClass24_0.<<ExecuteAsync>b__0>d.MoveNext()
```
