Adding previous difficulty retarget to the difficulty adjustment api. (#652)

refs #640

frontend/src/app/interfaces/websocket.interface.ts

@@ -10,6 +10,7 @@ export interface WebsocketResponse {
   mempoolInfo?: MempoolInfo;
   vBytesPerSecond?: number;
   lastDifficultyAdjustment?: number;
+  previousRetarget?: number;
   action?: string;
   data?: string[];
   tx?: Transaction;