Update SellerBoku in Solitude (bug 1002767)

- Check if a SellerBoku for a SellerProduct exists
- If it doesn't, create it
- If it does, update it
- If there are more than one, log and raise an exception
- Tests
Merge pull request #2047 from JaredKerim-Mozilla/1002767

Update SellerBoku in Solitude (bug 1002767)