Type Alias Invulnerables

Accounts whitelisted by governance to always submit their solutions.

They are different in that:

• They always pay a fixed deposit for submission, specified by [Config::InvulnerableDeposit]. They pay no page deposit.
• If ejected by better solution from [SortedScores], they will get their full deposit back.
• They always get their tx-fee back even if they are discarded.