* DB schema improvements
- add column DragonId to table PairData
- add foreign keys for table PairData
- remove unused column InventoryId from Vikings table
- simplifying relations viking -> selected dragon
- remove SelectedViking and SelectedVikingId from Dragon
- remove column SelectedVikingId from Dragons table
- cleanup and arrangement code in DBContext.cs
* support for dragons PairData in KeyValueService
and in "pairs" API endpoints