Add previous adjustment retarget. (#655)

* Add previous adjustment retarget.

* Fix green color.
Add + symbol to difficulty change.

* Add previousRetarget to websocket.

* Add previous retarget.

frontend/src/app/interfaces/node-api.interface.ts

@@ -25,3 +25,12 @@ export interface CpfpInfo {
   ancestors: Ancestor[];
   bestDescendant: BestDescendant | null;
 }
+
+export interface DifficultyAdjustment {
+  difficultyChange: number;
+  estimatedRetargetDate: number;
+  previousRetarget: number;
+  progressPercent: number;
+  remainingBlocks: number;
+  remainingTime: number;
+}