[FR] Set primary site per site group

[tibbis]

I have a use case where i have multiple site groups with sites and I don't want to point the x-default page to the same default site on all site group as it may not be related.

Site Group A

• Site 1 - Default
• Site 2
• Site 3

Site Group B

• Site 1 <- "Default-for-group-b"
• Site 2
• Site 3