Got serialization working for all level 1 test characters.

- Allowed MainHand and OffHand items to be null.
- Removed now unused schema, checks and tests.
Working on level serialization.

- Levels are not deserializing correctly. Need to fix.
- Changed Level03.OriginChoice to Level03.UtilityPowerOrigin as it is a
  more descriptive name.
More serialization work. Still failing.

- Fixed but in Item.Equals where derived types were considered equal.
- Removed TypeNameConverter in favor of the more secure OriginConverter.
- Changed CharacterJsonData to use Origins rather than type names for
  origin serialization.
- Made LevelXXJsonData properties required.
Converting from ItemJsonData to ItemConverter.

- Deserializing has not been converted.
- Renamed ItemJsonDataConverter to ItemConverter and changed types in
  CharacterJsonData appropriately.

Future work:

- Remove ItemJsonData and subclasses.
- Fix enum serialization (quotes are being added unnecessarily).
- Remove slot "None" for items.