Announce difficulty locked in on last block of epoch

frontend/src/app/components/difficulty/difficulty.component.html

@@ -33,6 +33,7 @@
                 repeatCount="indefinite"/>
             </rect>
           </svg>
+          <span *ngIf="lockedIn" class="lock-in-msg" i18n="difficulty-box.adjustment-locked-in">Difficulty adjustment locked in</span>
         </div>
         <div class="difficulty-stats">
           <div class="item">

frontend/src/app/components/difficulty/difficulty.component.scss

@@ -172,6 +172,18 @@
   width: 100%;
   height: 22px;
   margin-bottom: 12px;
+  position: relative;
+
+  .lock-in-msg {
+    position: absolute;
+    left: 0;
+    right: 0;
+    top: 0;
+    font-size: 12px;
+    line-height: 22px;
+    width: 100%;
+    text-align: center;
+  }
 }
 
 .epoch-blocks {

frontend/src/app/components/difficulty/difficulty.component.ts

@@ -54,6 +54,7 @@ export class DifficultyComponent implements OnInit {
   expectedHeight: number;
   expectedIndex: number;
   difference: number;
+  lockedIn: boolean = false;
   shapes: DiffShape[];
 
   tooltipPosition = { x: 0, y: 0 };
@@ -104,6 +105,7 @@ export class DifficultyComponent implements OnInit {
           this.currentIndex = this.currentHeight - this.epochStart;
           this.expectedIndex = Math.min(this.expectedHeight - this.epochStart, 2016) - 1;
           this.difference = this.currentIndex - this.expectedIndex;
+          this.lockedIn = this.currentIndex === 2015;
 
           this.shapes = [];
           this.shapes = this.shapes.concat(this.blocksToShapes(