Update `court` pallet

[maltekliemann]

Some of the changes to the court pallet are made in accordance to the recent changes to the specification of the DisputeApi:

• Let on_resolution return None if none of the jurors voted.
• Replace TreasuryPalletId with Slash trait. The purpose of this is to fix a simple design problem: Before this change, the court pallet used the TreasuryPalletId constant to calculate the treasury account. However, if the treasury pallet changes the way if calculates the treasury account, this breaks the court pallet. Instead, we now take a cue from the democracy pallet and replace the TreasuryPalletId with type Slash: OnUnbalance<...> and move unbalances created by slashing into the treasury using Slash::on_unbalance.
• Catch some errors in vote that were not considered earlier.
• Improve tests:
  • Replace unwrap() with assert_ok!.
  • Lower block amounts to shorten test duration.