enables pre-drag in GamePage
changes Board.unhighlightSquares to accept multiple highlight types
removes Board.clearSquareHighlights and changes unhighlightSquares to clear
all types if no types are specified.
corrects references to Board.squareHighlightTypes.NONE in Board.unhighlightSquares
updates clearSquareHighlights() to unhighlightSquares() in GamePage
puts correct username in Chat for draw offers (not the active player)
adds Premove - like Move but with less strict validity criteria (taking own pieces etc)