Tidy up DisputeView & fix broken comparator

Simplify and factor out a 'getDisputeGroupsByTrade()' method, to group
disputes by trade ID, from the 'show(Compact|Full)Report()' method
logic. Also fix a broken comparator on the Market column, that missed a
'CurrencyUtil::getCurrencyPair' application to the RHS parameter.

Additionally, do some minor cleanup of the class:

  Make fields final where possible, unbox primitive, convert statement
  to expression lambdas, simplify some if-else branches, use a loop in
  in place of 'forEach' to allow local variable mutation, formatting.