Improve impersonation: allow changing "app owner" token

[Sebazzz]

We currently have build impersonation. This allows easily switching account, but bypasses account security. Specifically two factor authentication is bypassed by impersonation.

Easiest way to resolve this is simply allowing a user to change its active "context" or "token": the current "AppOwner" group. This would be a safe alternative to actually logging into another account.

• Database changes to support this: Allowed AppOwner groups should be kept in a n:m relationship
• Current app owner should still be stored in database; on changing app token new login cookie with new token should be generated
• Add page for this
• Migrate existing impersonations to "app owner" token change
• Ensure SignalR / online detection still works properly